I calculated it to a female will lay on average a litter of 10 a month.
This is only possible if the male is in with the female 24/7 and she gets pregnant within a couple days of giving birth. No rest period between pregnancies will also wear out your females faster and they may not have as large of a litter or as healthy of babies.

When I bred rats, I wouldn't put the male and female back together until the babies were weaned (4 weeks) and my average litter size was 15 or so. I even had two different mothers have 20 babies each.