I think what is true is that if one person drove a gas spewing car, it wouldn't harm the world because it would dissipate. If a million people drive a car spewing gas, it soon becomes uncomfortable and toxic for everybody.

However, it's also possible that a million people can simply not drive. So while I'm aware of the population issue, I have never found anything information that satisfied me about the exact number the earth can comfortably support if we lived in a different way.