1. there is no way to tell she's pregnant. There are no visible signs until a couple weeks after the mom has given birth. At that point you will start to see baby bumps in her pouch and she is no longer pregnant, at that point it's called In pouch or joeys in pouch.
2 There really isn't an answer to this. Every glider is different.
Many things can signal when it's time to stop allowing a pair to breed. Temperament, Aggression, rejecting babies, eating babies, age, medical issues and so on.
It's not determined by how many babies they have produced.