You will want to stock as few fish as you can, while still providing a crowded enough environment to disperse the aggression and prevent any specific fish from becoming overly harassed. Remember, over-crowded aquariums can foul up rapidly, and events like a power outage, vacation, etc. can cause rapid deterioration of conditions. It will also create more work for you as the frequency of maintenance routines rise with the stocking levels.
Specific numbers will vary depending on what type of mbuna you choose. As freshmike has mentioned, we often have no choice but to keep the hyper-aggressive species in extremely high densities in order to prevent fatalities. On the other hand, for many mbuna, this type of dangerous overcrowding becomes less necessary. Physical size can also be a concern. Some mbuna barely reach three inches, while others can top more then eight.
I personally believe that it would be wiser to avoid the mentioned hyer-aggressive varieties unless you have a very large aquarium, this way space alone would help deal with the aggression, and crowding becomes less vital.
TCA