Eh. Overpopulation. I'm getting so sick of that word.
1) Eventually, all countries are bound to complete their demographic transition, something that the top 60 or so countries have already completed, and at that point there wont be any population growth. Demographic transitions occur with the rise of the development of a nation, or HDI, and as you can see
here, there were 3 countries in the world that had a development decline in year 2011, one of them being Libya for obvious reasons.
2) Humanity is sustainable in any numbers because of 2 reasons; technology and the fact that the more people you have, the more food is grown. When it comes to food, humanity is completely sustainable.
As for things like the atmosphere and its linkage with overpopulation... well, I'm sure scientists will figure out mass CO2 scrubbers fairly soon. All in all, overpopulation is a problem that receives way too much attention.