Cut out meat, suburban lawns, and the cars that go with them, and the earth could support 10B people right now, at the living standards of a typical Millenial in a cosmopolitan metropolis.

Unfortunately, there's pretty good indications that most people on earth want those meats, suburban lawns, and cars, so I think it's more likely we'll get war instead. But it's totally doable, at a standard of living that people will grumble at but could abide.