The "how many?" is a better question. After some googling I found that Humans can repopulate with around a minimum of 1,000 people. That way you can avoid and/or ignore things like interbreeding, infertility, stillborns etc.
It was noted though that Humans have unusally higher genetic diversity than any other Mass Effect race, so other races need less than 1,000. The less diverse the DNA the less people they need to produce a healthy population.
The other end of the scale is obviously the Asari. They already discourage mating with other Asari, since genetic traits and cultural insight is gained from mating outside their species, so it's considered wasteful for Asari to reproduce together. Also Ardat-Yakshi is only possible with purebloods. So they only need 1 Asari.
Rest of them probably falls in like this:
1: Asari
50-200: Krogan, they are notably reproduce fast.
100-300: Vorcha, they have low genetic variety because they adapt at an idividual level, not as a race.
400-600: Salarian, they reproduce with eggs. Unfertilized eggs produce males and fertilized eggs produce females. Social rules prevent all but a fraction from being fertilized. As a result, 90% of the species is male. Which means they need less females.
660-750: Elcor, Hanar, Volus, Drell, Batarian, Turian. There's no info about their genetics or reproducing so I put them 50-33% lower than Humans.
700-850: Quarian, same as above but they have a bad immune system.
1,000: Human.
Edit: Considering that the Collector Ship could house all human colonists you could easily take 10,000 Humans and 10x the rest of them as well on an Ark.