Mbuna are usually very densely populated in nature, so as long as you have plenty of rockwork in the tank and good filtration you could put about eight in a 75. It is better to put more of one kind of Mbuna rather than many different species. Mbuna usually are best in groups of four to six, with a ratio of one male to two or three females. Your tank will hold more than eight Mbuna comforatably, but if you start out with eight, you will most likely ind up with more, as the fish become of breeding age.