Boy mice are territorial, which means they usually won’t accept other boys into their space. They can be kept singly, so if you only want one mouse, then you want a boy. In my experience, a single boy bonds more strongly to you, and makes a fantastic, interactive pet. Boys tend to be bigger, and live longer than girls, but they also tend to smell a bit more too.
Boys can also be kept in pairs or trios as long as they are brothers, or were introduced at a young age. I have great success keeping my boys in groups, although there is an element of risk to it. See the next section for more information on keeping boys together.
Girl mice are social animals, so you must get two or more girl mice to live in a community cage. Unlike with boys, girls can accept new mice introduced at any time, although it does depend on the personality and dominance of the adults – some girls can be quite territorial as well, and take a while to warm up to new additions. Most pet owners seem to find they want girls – I think the fact that they smell less tips the tables. Because of this, girls are usually harder to acquire than boys, so you may find yourself waiting longer, especially if you want to buy a group of them.