In the wild, rabbits live in groups. This should be replicated as much as possible in captivity, and so no pet rabbit should be without a rabbit companion. A single rabbit will get extremely lonely, even if you are prepared to spend a lot of time with it. It is for this reason that a pair of rabbits is a very good option, as they are capable of keeping each other company.
How many rabbits you have is up to you - as long as none are lonely, and they all have enough space
The next question is: boys or girls? Well, one of each will generally make the best long-term companions. However, unless you want to breed them, you must get them neutered - an operation that your vet can perform. Remember - if you keep males and females together without having them neutered you must be prepared for a sudden increase in the number of rabbits you've got! Breeding is not an area that should be entered into lightly.
Two girls can be very competitive, and they may be a little more aggressive towards one another than a male-female pair would be. However, many owners report that neutering can reduce this aggression.
Two males can fight a little too, but can be a good combination, especially two brothers who have been reared together. If they do become aggressive towards one another after puberty (at about 3 months old) it may be necessary to have them neutered.