I see you have 31 in at the moment. I think you could easily fit 40+ in there.
As I am sure you are already aware, it is better to overstock on Mbuna to reduce aggression in the ranks!! Excellent filtration is required to counter this extra bioload but your tank looks crystal clear anyway!
Just a suggestion...add some more rocks (quite a lot more if you can) to make it appear more like your Mbunas' natural habitat. Caves and tunnels are good.
Can't wait to see some pics with your fish in it!!