It all depends on the number you get and what specific species of mbuna. Larger tanks are always better, but generally speaking I think would be ok with a couple mbunas in a 40 gallon. Keep in mind that they require special water parameters (pH and hardness). Really neat fish though.
Im not sure how big yours will get but as a rough rule you should follow this:
1 inch of fish to 1 gallon. So if you add up the size of all your mbunas and its less than 40 inches, your ok.
what mbunas do you have?