If your heart is set on Mbuna, and you only have access to the more common species, in a 40x20x20, you could probably get away with 1 male, and 6-7 female L. caeruleus without too much issue. If they are unsexable sizes, buy enough to weed out a few extra males as they mature. Give the females plenty of escape routes, and hidey holes. Feed sparingly, keep them small. In the wild adult males are typically a max of 3 - 3.75" TL, females 3" and under. Check out this past discussion for what wild labs look like.
Wild Caught Labidochromis caeruleus "Lion's Cove" | MonsterFishKeepers.com