They don't have to be kept in pairs but it seems like everyone who talks about them does, so maybe they are happier as pairs. I can't talk about personal experience because I've never had them on account of the fact they live such short lives.
I prefer them in pairs as they show off their 'social skills' better with their own kind. Just make sure that there is enough room for two fish to live separately if required as they can have a fair go at one another. they squabble all the time but nothing serious!
I agree with the other's make sure there is plenty of room for 2 rams if u get 2 that is cause every once in a while one will want to stay away from the other, but a while ago i seemed to have lost 1 of my angels and 1 of my rams, and the 1 angel and 1 ram that are left seem to have become buddies which is kinda finny cause the angel is 3-5 times the size of the Ram