20,000 Human colonists

Would be cool to see a BSG style colonist total at the end of major story sections.

Ha. Pretty much this.

And it's an exploration/colonization not an invasion.

Also, human history suggests that arriving on the shores of occupied territory can often end badly for one party or the other.

So whether the number is 2,000, 20,000 or 200,000 probably doesn't make much difference to the plot 

Ikr. Seeing pathfinder killing turian bandits and turians becoming bandits seems off. I mean what if we as the pathfinder kill 1000 bandits of Milky Way galaxy origins? Unlesss cloning is something viable I doubt this small group of colonists will survive that long across multiple worlds. Seems illogical to me that the number is this low.

Maybe because the selection of the Arks was extremely bad? I mean, we're talking a bunch of people that thought an Alliance officer with disciplinary problems would make for an excellent chief of security.

They pour gigantic effort and funds into AI, but don't have the patience to gather up appropriate crew members with verified background?
I find that hard to believe.

So poor logic and thought are added into the mix of the andromeda initiative....more bad bioware writing.

It's a high enough number for genetic viability, at least assuming some cross pollination, so to speak, between colonies.  Also, all this is limited to a few local clusters because there are no mass relays so everybody is in the same general neighborhood which helps with transfer of people and resources.  They are also limited then to whatever garden or easily terraformed worlds are in that local cluster which I would hazard isn't really that many.  Fighting the Kett is more problematic, but we know virtually nothing about them at this point.  It's entirely possible they aren't terribly numerous or that their tech isn't very good.  If this game is thematically about the grey ethics of colonization (which appears likely) they could easily be the Maori/Aborigines/American Indians/etc. to this story.  The fact that Bioware is apparently going for an "organic" or vaguely earthy theme with them sort of buys into this theory.  In certain scenarios Cortez only needs like 500 guys and the help of some disaffected locals to topple an empire.     

Now I do agree that giants ships capable of sustaining that many people for centuries makes no sense but that kind of techno magic is just something that has to be for the story to work.  Like eezo. 

Yes, which is why I expect more of a "one charismatic upriser and the rest will follow" kinda of event.
The Sloan Kelly (?) (chief of security) might simply be too much of an individual and/hot head whom many other members of AI who are more "in favor or more action oriented approach" may like to follow.

Unless she "kept it low", until they arrived to Andromeda...

Yes they would need to breed like rabbits and women would not at all be placed in combat roles due to risk of death in the battlefield. But this is bioware and female representation is 50/50 for combat roles even though it shouldn't for this scenario since they should be breeders.