how many people is too many people?

A 2010 study published in the Proceedings of the National Academy of the Sciences looked at the link between policies that help women plan pregnancies and family size and global emissions (the study also looked at aging and urbanization trends).

The researchers predicted that lower population growth could provide benefits equivalent to between 16 and 29 percent of the emissions reduction needed to avoid a 2 degrees Celsius warming by 2050, the warning line set by international scientists.
The Atlantic, 1 Nov 2014