The male/female ratio will depend on the specific species. With labs, I'd throw in a group of 5-6 and you wouldn't have to worry about the m/f ratio likely at all. With a lot of mbuna species, a ratio of 1m/2-3f is best. With very aggressive species like M. lombardoi (kenyi), I'd recommend no less than 1m/4-5f. And demasoni specifically, the actual m/f ratio doesn't matter so much as having an appropriate-sized group (at least 10-12).