[fmueller]

I'd say you can have more than 12 mbuna in a 55G. In fact more would be better to spread around aggression. However, I would second the recommendation to limit yourself to one or two species. That will also help with aggression control. If it's all about color, consider species like P. saulosi - with blue males and yellow females you have two colors covered with one species! Plus saulosi are smaller and less aggressive than most mbuna.

I find this 400G single species tank with P. saulosi by CF member Ssssssspit_Fire absolutely stunning!