How many mice should I get?

At least two - Mice are social animals and it is *very* unnatural for them to live alone. Single mice are generally bored, lonely and miserable, and they do not make good pets as they tend to spend more time sleeping or hiding in the nestbox than mice kept in groups.

With a single mouse you also miss out on the fun of watching mice play together and groom each other.

It is really not fair to mice to keep them alone if it can be avoided. 

If they liked solitude,they woudn't live in colonies in the wild.

Sometimes male mice which are very aggressive towards other males have to be kept alone.