I would never suggest more then 20 full size mbuna but I've had much success keeping 40 dwarf mbuna in a 55. I know that people say secessful tanks should only be considered at 1 year but the tanks been up for 6 months and I only had one weird week when they were scared of me. Other then that everything is perfect I've had 0 deaths and I got cute little fry in a 10 from them. Also 20 6in fish is 120in total and 40 3in fish is 120in total I know it doesn't work that way with mbuna but it makes it sound not as outrageousdemasonisoldier said:People thought I was crazy but the best set up I had in a 55 gallon was between 75 to 100 Melanochromis parallelus. These varied in size from the Apha male was 5 inch and there was about 15 this size. The others where 3 to 4 inches with about 30 1 inch brood. This tank had the least aggression out of the many Cichlid Combos I had. Needless to say I have a mixed Malawi tank with about 15 fish and keep having fatalities. I find the more fish the better. I had plenty of rocks and a couple of pottery caves. The bottom was little pond rocks I got a Lowes which proved to allow hundreds of babies to survive and be sold and between 1 to 1.5 inch. In this tank I think I have about 3 females breeding and the one alpha male. You can see pics on some of them on Cichlids dot com under the name Jon Brown and Melanochromis parallelus.
Just make sure you have really good filters. I think my water was filtered the aquarium 15 times per hour and I changed the water at least once every 2 weeks to keep it clean. I know many people will say different thing but I think at least 30 cichlids minimum.