Apparently the number is less than 5 billion if we’re talking about current way of life vs resources. Right now we’re using up more resources than the planet can produce, meaning we’re already on the road to destruction.
We should either:
1) use less (food, energy, oxygen etc)
2) have fewer children so that the population goes down
3) have a nice little war and reduce our numbers by a couple of billion. Genocide against either the Indians or the Chinese might possibly solve the issue. Though Americans seem to use more resources than anyone else, so perhaps we should do that instead.