2) No decorations and as many fish as will fit. 20 would not be too many.
The reasoning is, if there are decorations, there will be territories. If mbuna have territories, they will defend them. If there are not enough spots for each fish, they will slaughter one another. This is where africans get their bad reputations from (that and some are just naturally pshyco killers - ask for species-specific information later).
Both are doable, though many people will argue option #2 is poor animal husbandry. It can make a stunning display, though.