Without knowing specific species you're considering, it's impossible to say. If you went with a single-species tank of a fairly docile fish, say Labidochromis caeruleus (usually called yellow labs or electric yellows), you could go with 7 or 8. More aggressive or territorial fish, e.g., Pseudeotropheus lombardoi (aka 'kennyi') wouldn't be appropriate at all.
Have you any likely suspects for the tank?
Jim