Its noteasy to simply recomend the number of fish that will fit into a tank. Each fish has individual requirments. Tey also have different eatng habits and differing amounts of waste material from them. There are relatively small fish that give off quite high waste products and thus you need to be careful on the strain you place on the filtration. There are also some quite large fish that need very little space and/or small productrs due to their very nature.
Basically if we look at the standard fish that most people keep then i would suggest clowns, dottybacks, chromis, small wrasse dwarf and pygmy angels.