It is hard to set a definitive number on this because there are to many factors to take into consideration, some that can be measured and some that can not. The answer to this question needs to be based on the premise that resources are limited, therefore they need to be carefully distributed among the global population without distroying earth. Setting a definitive number would imply that all people have the same needs and habits, making it possible to calculate the average resources consumed per capita. A popular example is the one that states that if the whole human population cunsumed as the average american does, four Earths would be needed.
Other factor to take into consideration is the speed in which resources are regenerating. If they are not regenerating at all, it would be an indicator that shows Earth is dying and the population or the levels of consumption should therefore reduce.