That is a question impossible to answer. I offer the speculation from what information the Mass Effect works have established:
- Earth has an human population of 11.4 billion, making it the most populated planet known to the galaxy.
- Sur'Kesh has an salarian population of 10.3 billion;
- Irune has an volus population of 8.8 billion.
- Palaven has an turian population of 6.1 billion.
- Thessia has an asari population of 5.5 billion.
- Dekuuna has an elcor population of 2.35 billion.
- Tuchanka has an krogan population of 2.1 billion, though quickly dwindling because of the genophage. Estimatedly 1 billion of the krogan on Tuchanka are female.
- Rannoch has an geth population estimated between tens of millions to the single-digit billions physical platforms.
- The Migrant Fleet has an quarian population of roughly 17 million; they one-child per family rule; their population will remain consistant.
- The batarian population is not known; they have a fair amount of space and are most likely in the billions.
- Kahje's hanar population is not know and the drell living on their planet, because their own no longer support life, have a rough population of 300.000 people.
But the above is not the end tally, Earth merely has a higher population because we have only recently began colonizing. Notable colonies that has become symbols for humanity, like Eden Prime with roughly 4 million people, Terra Nova with also roughly 4 million people, Bekenstein with a 5 million population, are still growing and as a result most humans are still on Earth where most of the other races would have moved from their home-world to their colonies to even out the population.
While the hanar population is not known, many Earth jellyfish species reproduce extraordinarily quickly; roughly 10.000 thousand new eggs per day that will become a jellyfish is sperminated. Most likely the hanar population has no biological limit to surpass the other species but rather social rules prevent over-breeding because of their home-planet's limited resources and similar limitations with colonies. But, at least in theory because of similiarity, the hanar would have the greatest population.
Human population increase with 100 million per year, today, because of the similarities between asari, batarians, and turians, we assume all of these have the potential to increase their population with similar numbers.
Because of asari's long life-span they would slowly gain a greater population which allows the other species to accelerate beyond them however they also prevent other species from having children of their own which could compensate.
A salarian female can lay a clutch of a dozens of eggs in one years time but only 10% of the species are females, about one billion salarians of the 10 billion living on Sur'Kesh are female, directly limiting them.
While the krogan race have few numbers and are directly limited by the genophage, krogan females had the ability to produce clutches of up to 1,000 fertilized eggs over the course of one year. But no longer.
It would seem that either salarians, asari, humans, turians, batarians, or hanar, have the greatest populations. If the krogan's genophage is cured then they may soon have the greatest population, especially because hanar would be limited to certain worlds with the proper enviroment where krogan would not. --Theh5 (talk) 02:38, July 12, 2017 (UTC)