Quite a while back, even maybe 10 years ago, I read online that the planet Earth can sustain only 1.9 billion people on it to be fully self sufficient.
That 1.9 billion number has stuck with me all these years and I always think about how we are 4 times over that number and still not respecting the scarcity of resources at all.
I did a quick google research and it says now that the Earth could sustain 4 billion people at a reasonable standard of living, but only 1.5 billion people at an American standard of living.