I usually keep between 4 and 6 mice at a time in my 10 gallon tank. I gave them a little house a paper towel tube and an excersise wheel I figure they might as well enjoy there stay before they meet their end.
If you get litter mates you should be fine but if you introduce male mice that are not litter mates they may fight.
I usually get 4 to 6 and work my way down to none then buy 4 to 6 more litter mates and I have not had any problems yet.