in a 20 gallons, I would start off with 3 pairs and see what happens.. It all depends on what else is in the tank and I always got equal ratios of males to females. Rams will pair off with one mate and do not tend to take on two mates like some apistos do.
Get 3 males and 3 females in there and you will notice that 20 gallons is too small once you get a pair cause they will stake out a territory. Its better if the tank isnt a bare bottom and has plants and driftwood in it. at the most you will have 2 pairs form up and defend a territory but that will cause alot of problems for the remaining fish. Be prepared to move the rest or give them away..
If you wanna breed then move the mated pair to a BB 10 gallon tank with a piece of slate and Voila!!!!
Check out Simply Cichlids, we have a library with an article on breeding rams