Gestation of a mouse is 18-24 days (depending on your source).
Breeding is instanteous when adding a male in with females. Females come into heat every couple of days.
It is best to have ‘friends’ together. Female mice will raise babies better when with friends. Don’t add a fully pregnant mouse to an established colony.
Remove male after 16 days. Causes less stress to the mom. And leave him out until her litter has been weaned for 2-3 weeks. She needs to rebuild her strength as well.
Crowding reduces litter size.
Over breeding reduces litter size.
Handle them frequently but don’t disturb a new litter for the first couple of days. Let mom get established. Mind you, I find this less of a problem if you have an experienced momma as part of a colony.