300 Million Americans
“More! More! I'm still not satisfied!”—Professor Tom Lehrer
On the other hand …
What if the environmentalists happen to be right, for once?
If there is a limit to the number of people on Earth, we can expect rents to rise. Higher rents usually go along with declining birth rates. (There is a problem with this analysis. If enough people underestimate the limit, they might not be surprised when rents start increasing even if that increase was due to political corruption.)
Even if we look beyond Earth to the rest of the universe, population growth might have a subexponential limit. Rents might be enough to slow growth without stopping. If enough people are travelling close to the speed of light looking for cheaper neighborhoods, time dilation can slow population growth.
Environmentalists have tried getting around similar arguments by inventing the theory of “overshoot.” (This is partly due to the potato chip from Brazil phenomenon.) They have some empirical evidence that animal populations likely to overshoot, but we're plants.
On the gripping hand …
The environmentalists need not be right. It might be possible to create “basement universes.” There might even be technical fixes so innovative that we haven't even thought of their possibility.