Davy I use groups of 1.6 mice like Kerby mentioned also. A container with the floor space of about a 10 gallon aquarium is a good size for 1.6 groups. If you are using smaller sized cages, groups of 1.3 will work fine also.
Something interesting to mention. Over the years I have held back MANY young mice to rotate in as new breeders. Of all the mice I held back to breed I have never had a male in a group of 1.6 that didn't breed and reproduce well. I have heard of some people using more than one male to be sure they have a fertile male but I haven't ever seen one that wasn't.
I don't ever seperate pregnant females out of their group either. . Females eating their young happens so seldom with mice that it's not worth the effort of seperating them. As someone else mentioned, other females in the group will help care for other females babies. You will often have 3 or 4 litters that were born at different times being kept in a large mass in one corner of the cage and being cared for by all the mothers together.
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Randy Whittington

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