I’d think it wasn’t possible to get enough people off-world for a viable breeding population, but that number is MUCH lower than I thought it was.

newscientist.com

Of course a gene-pool that small would probably lead to Martians looking quite a bit different from people on Earth.

I still think it would be a lot easier to establish colonies in bunkers on earth to survive a disaster, than it would be off-world. Even with a damaged atmosphere, this planet has exactly the right gravity for life as we know it to thrive in.