Hi - it depends really if you want the bunnies to bond together or with you.
If for example, you're around at home a lot you may only want to get one so it bonds with you. But if you work full-time etc it is nicer for the bunny to have a buddy during the day.
I had two bunnies (M+F) who bonded amazingly, but to the point where they really werent interested in me at all, and if I took one away to pat etc, the other bunny would look on at me angrily, jealously!
I now have 3 new bunnies (littermates) 2 males and 1 female, they get along amazingly well, its really cute. and because there's three they seem to alternate who they're bonding with, with means the 3rd bunny is always happy to be petted/be friendly with me.
If the bunnies are neutered I think any combination of M+F or M+M is fine. Probably also F+F although I'm not experienced in having 2 girls, I guess that would be ok too once hormones out of the way