Thanks to Google...
Rats can reach sexual maturity at 5 weeks of age....Rats do not have a breeding season, although very hot or cold temperatures will reduce breeding. Females of breeding age come into heat all year-round, every 4 to 5 days, unless they are pregnant, and even then, they may come in heat once or twice early in the pregnancy. Each female usually has a regular schedule that can be marked on the calendar, but it can vary. Each heat usually begins in the evening and lasts most of the night.
...In rats, the average litter size is 6-13 pups
Rats can reach sexual maturity at 5 weeks of age....Rats do not have a breeding season, although very hot or cold temperatures will reduce breeding. Females of breeding age come into heat all year-round, every 4 to 5 days, unless they are pregnant, and even then, they may come in heat once or twice early in the pregnancy. Each female usually has a regular schedule that can be marked on the calendar, but it can vary. Each heat usually begins in the evening and lasts most of the night.
...In rats, the average litter size is 6-13 pups