Well, i've heard that you can have 15 small ones in a 55G.
My 55G has 8 in there right now, and its looking pretty full, and none of my fish are even adults yet!
So it depends on what cichlids your intrested in.
Do you have any idea on what you want to put ion there? Because, not all African cichlids are compatable. In fact, there are a few that aren't.
Depending what you get is the real answer. Demasoni only reach about 3" and Crabro will reach 8", so obviously the stocking levels would be quite difference. For the average 5" mbuna I agree that you're looking at about 12 or so.
There are many that aren't, stock carefully and don't impulse shop. Keep fish of similar aggression level, in appropriate mf ratios, and choose species that don't look anything like each other.