Assuming this is a 48" tank, you can do 15-20 demasoni and a regular sized cluster (4-6) of a non-dwarf mbuna. That's what I keep in a 55gall.
To the extent you're buying them small and going to sex a few out of one or more clusters, you can fit a few more when they're young.
It's my view that in particular, my non-dwarf mbuna are not thrilled with the 55 and if I could fit it, would probably like something larger... but they are fine, and the demasoni are genuinely comfortable in that size tank.