Hmm, figuring out just how many takes math...something I am horrid at!

However, I can say that the harem method of breeding (one male with two or more females in a tub) works much better with mice since you aren't changing things around all the time by moving the males. Mice like their groups to be steady and they prefer to be left alone most of the time...at least they seem to breed better when those things are done.

Good luck setting up the colony, and remember that it takes a month or more to get them going! Hopefully someone with better number skills than I can tell you how many to start with.