2/3 billion at Euro standard of living? This isn't what I've read.
2-3 billion could be supported at a 1920s-40s standard of living. 2 billion if you want to keep some plants and animals alive which aren't useful to humans. 3 billion if we use every last acre to meet human needs.
This is if you are looking at
Ecological Footprint.
The only way we could support 6+ billion of us is if we all lived like monks and ate beans every day. Since that is not going to happen, our numbers are going to be reduced. If we do not reduce our numbers voluntarily than nature will do it for us through war, starvation, and disease.