I'm pretty sure your estimation of a litter a month per female is off.

I've noticed that the girls I have tend to pop at 2 month intervals, not monthly.

I keep 1.4 and 1.5 and 1.6 because I want to check total output for my location and set up. The girls that are nursing generally do not get pregnant until the babies are weaned but I think that is because the other girls who are not nursing do get pregnant.

There are always one or two pregnant rats in every tub.