How many 4-5" cichlids can be placed into a 55gl take before it's considered stocked?
It really depends on their aggression level. If you stick to peaceful species (by mbuna standards), you could have up to 15-16 fish; however if you go for the more aggressive species, you'll want to stick to around 12. The reason for that is that more aggressive fish tend to take more of the tank as territory, and don't play nice with their neighbors, while the more "peaceful" will tolerate others with minimal fuss.
Also, IME having fewer species with more individuals works out much better than having lots of species with only 2-3 individuals.