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.
I recall reading something similar.
I guess "standard of living" is subjective depending on location. Some of the conditions I've heard about in some of the South American and African countries would be considered way, way way below Australian standards and I think the people at both ends of the scale are responsible for what's going on, albeit for different reasons. What's common to all is wastage, greed and sometimes very little care-factor.