For the proper rotation you need 1 female rat per snake.

You should avoid breeding rats in harem because it takes it's toll rapidly on females since they are pregnant and nursing, litters size become smaller so are the babies which grow at a smaller rate too.

For proper rotation which means producing consistently every week you have to consider that

2 females a week will produce 12 to 24 babies a week the process will take 3 to 4 weeks between breeding and gestation + an additional 3 weeks before the animals are weaned that is 6 to 7 weeks if you need smaller feeders you can reduce that to 5 to 6 before your female goes back in rotation.