I've been disappointed over the decades to see the word "population" largely disappear from the mouths and writings of environmentalists (which technically includes everyone who wants to stay alive on the planet, however somnolent that inner environmentalist may be).
I know that Paul Erlich's Population Bomb has been criticized by many. I recall an article titled "The Population Firecracker," long ago and not long after the book came out, I think by William Buckley. However, population goes to the heart of the vast majority of problems we have today, from war and pollution to climate change. It also strikes me that both Erlich (and his wife who wrote the book with him) and Malthus were 100% right, but for the timing. The deductions they made from what was observable at the time were completely logical, but none could foresee the technological changes (often a difficulty when projecting trends into destiny).
Thus I was pleased to see climatologist Dr. Paul Beckwith come out with this video not long ago. He makes points near the end of this 15 minute video regarding Canadian immigration problems, which upon reflection I believe are worth some serious thought (recognizing of course that in our own case it is we who created so many immigrants, not to mention that we Europeans were the latest immigrants here ourselves).
I think instead of asking, "How many humans can the earth hold?" a more proper question is how many should it hold? As many troubles as we have now, it is even more troubling to think of an earth with nothing but humans, starlings, cockroaches, rats, and maybe some weeds thrown in. Anyone who's raised rats or gerbils well knows how far south things can go in a short time, with behaviors rapidly becoming antagonistic, even fatal. We aren't that different in our territorial tendencies and resource needs.