That's a tough question, because the number of babies per litter varies between litters and mothers. You are going to need about 25 rodents per week. Personally, I would shoot for a surplus. That allows you to have some in the freezer saved up for when your production levels hit a bump in the road (which has happened to me more than once). If you assume one litter per month containing 10 babies each, you would need 10 female breeder mice. I would use 15 so that there is no down time when I am retiring them.
good luck
Denver Tom