For me i retire every year both rats and mice , irrelevant whether they are dropping 5 or 15 babies, I replace rack by rack over the course of our winter each rack has 96 females and 24 males (4/1 ratio) Mice 5/1
At this stage i have 12racks x24 tubs (Rats) = 1152 females + 288 males
Mice = 1000 females + 250 males

All are changed in a three month Window (our Winter where sales are the lowest ) so that when summer starts i have the females already dropping there 1st litters