Its not an ideal size for them but you are probably looking at 8 or so fish for an all male set-up, and you will have to choose species very, very carefully (and ideally have a plan B to sort out problem fish - either another tank or a return policy).
How long is the tank, what filtration do you have available ?
It is a risky proposition for a set-up, you will probably lose some due to aggression, and a lot of people will say that a four foot long tank is minimum size for lake malawis.
I'd probably steer you towards tanganyikans for this tank, if you do not have prior experience of africans.