How many pet mice should I get?

Mice are more social than other small animals (like hamsters) but that doesn't mean that they won't necessarily fight if you have too many. They could fight over food or space, or just because they don't like each other. Remember animals have different personalities just like people. Since you have a larger tank (20 gal right?) You should be able to get 4 or 5 just fine with no problems. ^_^ I had 4 female mice in a 20 gallon and they got along just fine. Also remember that since they are still young there is a chance that they might fight as they get older. Hope that helps!