OK, if you get two clowns that are smaller (juveniles), you will eventually be assured of having a pair. They will squabble out for a bit (entertaining to watch at times) and decide on their own which is the dominant female. The other will become the male and POOF! you have a pair!
Buying mated pairs are more expensive, and not always true. Many not-so-honest dealers will sell you a "pair" when really they are just two larger clowns. The risk is getting two females as they will fight and one will eventually loose and die.
I disagree with getting four of them. Yes, you will eventually get a pair, but it will take longer as there are more of them so have to sort out dominance. Also, the two least dominant will be harassed by the dominant female and male to the point you will have to remove them.