Evidence in the text that shows couples are only allowed to have at most 2 kids.

It was the government's idea, they were the ones who authorized it-- how else could a Third like Ender have got into school? And now the
monitor was gone. The experiment entitled Andrew Wiggin hadn't worked out alter all. If they could, he was sure they would like to rescind the waivers that had allowed him to be born at all. Didn't work, so erase the experiment.