In the Mass Effect universe, the galaxy is positively teeming with a diverse collection of alien species. Although humans are the only playable race, Commander Shepard still has ample opportunity to rub shoulders with other lifeforms inhabiting the Milky Way galaxy, such as Krogan, Asari, Turians, and Salarians.
The setting is so big that players may wonder just how many people actually inhabit the universe, aliens included. The lore runs pretty deep in the Mass Effect games, so discovering this information isn't always terribly difficult (most of the time). With some careful reading of the in-game Codex as well as character dialogue, details like population numbers can be confirmed without too much trouble.
20 Humans
Humans are the newcomers to the world of intergalactic relations. Shortly after discovering the remains of Prothean technology on Mars, humans were able to conduct space travel through the mass relays. This ultimately led to their first contact with the other races of the Milky Way.
Earth's population stands at about 11.4 billion people. In fact, it was partly due to the overcrowding experienced on Earth that led to a general desire to explore new territory beyond the planet's borders. In addition to Earth's population, it's also estimated that c. 250,000 humans inhabit the various space stations scattered across the galaxy. Other human populations, such as those living on Earth's far-flung colonies, are difficult to nail down with absolute certainty.
19 Turians
Turians possess a highly militaristic society with an intense focus on discipline. Their predilection towards public service, colonization, and willingness to win any conflict at all costs makes their culture appear much like the ancient Romans. Turians maintain a rigid code of honor that encourages them to be decisive and responsible for their actions.
The Turian homeworld, Palaven, is located in the Apien Crest. The population on this planet is estimated to be 6.1 billion inhabitants. Another 350,000 Turians are spread across myriad orbital space stations. Whether or not there are more Turians in the galaxy than these numbers suggest is a matter of conjecture.
18 Salarians
Salarians are arguably the most inventive of all the Citadel races. They are described as having incredibly fast metabolisms which allow their minds to work at a frenetic pace. They are primarily thinkers rather than pure warriors. Technology is the main area of Salarian expertise. The genophage, for example, was a biological weapon that rendered the Krogan sterile.
Sur'Kesh is the Salarian homeworld, a planet teeming with jungles and diverse species of flora and fauna. The population of Sur'Kesh is listed as 10.3 billion, with another 1.1 million Salarians inhabiting orbital space stations.
17 Asari
The Asari are a well-respected race in the Milky Way galaxy. Whereas the Turians and Salarians are noted for their toughness and intelligence respectively, Asari are renowned for their diplomatic skills. The Asari are also unique in that they were one of the first species to discover interstellar flight after the Protheans.
The Asari homeworld is known as Thessia, a planet sometimes referred to as "the crown jewel of the galaxy." About 5.5 billion Asari occupy Thessia. Another 33,000 live on various installations throughout space. Considering their aptitude for exploration, more Asari may be located in the further reaches of the galaxy.
16 Quarians
Quarians are a race of humanoids that protect themselves by wearing elaborate environmental suits. Like Salarians, the Quarians are proficient with technology, although the latter's focus generally tends to be artificial intelligence. Quarians were responsible for the creation of the Geth, a species of A.I. who ultimately overthrew their masters.
The cataclysmic war with the Geth left the Quarian population devastated. Their original homeworld, Rannoch, is currently occupied by the Geth. Quarians are now reduced to living as nomads. A vast collection of starships, known as the Migrant Fleet, serves as their home. Around 17 million Quarians inhabit the Migrant Fleet.
15 Krogan
The Krogan are a species that shouldn't be trifled with lightly. Generations of extreme hardship have tempered the Krogan into highly dangerous warriors. Krogan culture is very tribal and decentralized. Various clans hold fluctuating degrees of influence and power that often results in competition and rivalries.
The diminished status of the Krogan owes much to the mutations caused by the genophage. The Krogan were sterilized on a mass scale, making it difficult for them to reproduce. Only about 2.1 billion Krogan remain on their homeworld of Tuchanka. Another 2,400 occupy the orbital battlestations around the planet.
14 Drell
Compared to the other species in the Milky Way, the player doesn't typically come across many Drell. They are reptilian humanoids who possess thick muscle tissues and can secrete mild venom from their skin. One of their most distinguishing features, however, are their photographic memories, which are so vivid they can sometimes take the form of unintentional flashbacks.
Similar to the Quarians and Krogan, the Drell population has experienced severe depletion. The Drell homeworld of Rakhana was eventually rendered uninhabitable due to unregulated industrial expansion. Only about 375,000 Drell managed to be rescued by the Hanar before the planet's complete collapse.
13 Keepers
The Keepers are one of the more unusual and mysterious creatures Commander Shepard comes across. Their appearance is that of an insectoid whose only job seems to be the maintenance of the Citadel. Keepers don't communicate with other races and are extremely docile.
It's later revealed that the Keepers serve a much darker purpose. Their job is to keep the Citadel in working condition so that other races continue to settle there. Once these species reach the requisite level of technological development, the Keepers send a signal to the Reapers to commence their invasion. The exact number of Keepers is unknown, but it's suggested they number in the thousands, spread across the entire Citadel.
12 Elcor
The Elcor are a curious race in that their massive size belies their cautious nature. Evolution has rendered them as slow-moving and deliberate creatures, where even a small fall can potentially be lethal. Their speech patterns also reflect their calculating nature.
Dekuuna, the Elcor homeworld, is quite small in comparison to other species. The planet's population is recorded at 2.35 billion. Players don't come across many Elcor throughout their adventures, which implies that most of them are located on their home planet.
11 Batarians
Not every race in the Mass Effect trilogy likes playing by the rules. The Batarians fit the role of renegades quite nicely. Conspicuous by their four eyes, Batarians are an isolationist species who enjoy dabbling in the more illegal trades, such as slavery.
Khar'shan, the planet Batarians call home, is steeped in mystery. This is due to the sizeable amount of misinformation the Batarian government leaks to the rest of the galaxy. As such, the claim that 15 billion inhabitants live on Khar'shan should be taken with a grain of salt.
10 Volus
The Volus are a race that predominantly consists of merchants. They're easy to spot considering their short statures and ubiquitous breathing apparatuses. Although their deft understanding of economics is highly respected, they are not yet members of the Citadel Council.
The Volus hail from the planet Irune and are actually client subjects of the Turians. The planet's atmosphere consists of an ammonia-based biochemistry, which forces the Volus to wear their distinctive protective suits whenever they are off-world. The Volus population on Irune stands at about 8.8 billion with a few million more potentially spread across the galaxy.
9 Geth
Geth occupy a unique place among the other races of the Milky Way. Created by the Quarians as servants, the Geth are an A.I. collective that gradually gained sentience. In time, the Geth violently overthrew their former masters, almost annihilating the Quarian population in the process.
The Geth now control the Quarian homeworld of Rannoch. It's difficult to estimate exactly how many Geth are in existence, but it may be fair to assume they number in the millions. High-end estimates approach the low billions.
8 Vorcha
Like the Batarians, the Vorcha have acquired a seedy reputation among the denizens of the Milky Way galaxy. Their unusual biology allows them to adapt easily to different environments. Many Vorcha prefer mercenary work, which has done nothing to improve their standing with the Citadel races.
The Vorcha homeworld of Heshtok is a nightmarish landscape of volcanic eruptions and virtually no ecology. There is no single, centralized government either, making reliable population numbers difficult to acquire. Nevertheless, census estimates give Heshtok a total population of somewhere between 6-9.3 billion Vorcha.
7 Hanar
One of the few non-bipedal races in the galaxy, the aquatic Hanar resemble jellyfish. Unable to support their own weight, they utilize small mass effect fields to keep themselves levitated above the ground. Hanar are noted for being deeply religious. They worship the Protheans, who they refer to as the "Enkindlers."
Kahje is the planet the Hanar call home. Over 90% of the planet consists of water. The Hanar aptly calls this vast ocean the Encompassing. Since it's almost nigh impossible for other species to live on this planet, very little is known concerning population numbers. Considering their status as one of the Citadel races, it may be fair to assume the Hanar number in the billions.
6 Yahg
The Yahg hold the distinction of being perhaps the ugliest of the Mass Effect races. They are hulking, violent predators who are known to be extremely aggressive. As far as the Citadel Council is concerned, the Yahg are pariahs who should be avoided wherever possible.
It's mainly due to the Yahg's outlaw status that very little is known about their homeworld, Parnack. Thus, their population is impossible to know with any certainty. The only significant Yahg players meet on their travels is the elusive Shadow Broker.
5 Collectors
The Collectors serve as the primary antagonists of Mass Effect 2. They are a highly mysterious race once thought to be pure myth. Technologically, the Collectors are highly advanced, often willing to trade their useful knowledge in return for living specimens.
Their efforts to abduct human colonists form the basis of Mass Effect 2's main plot. Although hard-and-fast numbers are impossible to know, the volume of troops that are continually thrown at Commander Shepard suggests the Collectors are quite numerous, perhaps numbering in the millions.
4 Reapers
The Reapers represent the greatest threat to all living organisms in the Milky Way. Reapers are organic/synthetic hybrids who slumber in the dark recesses of space until they deem the time is right to emerge. They bide their time until the other races have achieved the requisite level of civilization before destroying them.
The much-anticipated Reaper invasion is launched in Mass Effect 3. The exact number of Reapers is hotly contested, with fans typically citing a number somewhere in the ballpark of 20,000 at the low end.
3 Protheans
By the time of the first Mass Effect game, the Protheans have already been extinct for thousands of years. However, their empire and the artifacts they left behind loom large in the trilogy's setting. The Reapers utterly destroyed the Prothean civilization, leaving behind nothing but ruins.
There is one exception. In Mass Effect 3, Commander Shepard can actually come across the only surviving Prothean left in the galaxy. Javik, a Prothean warrior who was preserved in stasis, is the only living example left of his race. As such, the Prothean population is composed of a single digit.
2 Leviathans
The Leviathans are deserving of their name. They are massive, Lovecraftian ocean-dwellers who have successfully managed to evade the Reapers for millions of years. Their origins predate recorded history and it's clear they consider themselves the galaxy's most advanced race.
Commander Shepard encounters a handful of Leviathans in Mass Effect 3, although it's difficult to determine just how many of these squid-like beings exist. The fact that they have been in hiding for so long may suggest their numbers are quite small compared to the rest of the galaxy's species.
1 Rachni
The Rachni are an insectoid species once believed extinct. According to in-game lore, the Rachni at one time posed a substantial threat to Citadel Space, which ultimately led to the Rachni Wars. It was during this conflict that they were nearly eradicated at the hands of the Krogan.
The Rachni weren't completely annihilated, being driven into hiding instead. Commander Shepard discovers the remnants of the Rachni in the original Mass Effect. The player's dealings with this insect race can play a role in how the Reaper threat is met in Mass Effect 3.