55g is the minimum recommended for any cichlid setup. If you want oscars you could have 1, some would say two but you will need to move once they get too big. 55g is great for smaller african cichlids, like mbuna which are very colorful. You could also go with discus as long as you change water more frequently or even other smaller new world cichlids. Its all up to you but just find out which type you want then decide how many from there.