It is such an unpopular opinion but this is the answer. Humans are selfish. The earth can't sustain 8B+ people. Maybe 2B max with some technological advances for cleaner cars and other renewables.
Anyway, earth could very well sustain even 20 billion people according to Martin Rees (in "On the Future"). It does require that we really reduce our resource usage, however. Going vegan, using virtual reality instead of travelling etc.
Luckily the trends seem to indicate we won't reach more than 11 billion (which is still way too much at today's resource usage) before the numbers go down.
Heck, a trillion or more, if we all agree to be suspended in tanks and fed by tubes!
The earth population that can enjoy life is not fixed at 2B, that's very myopic thinking. We should fund science & tech so that it's possible for 11B people to enjoy life without damaging the planet, instead of blaming the "others" based on racist anti-science attitudes and deliberately keeping them poor by punishing them economically.
https://www.youtube.com/watch?v=nzBAxcJDSsc
https://www.youtube.com/watch?v=QsBT5EQt348
http://www.overpopulation.world/