It does depend on the scenario, after one of our earlier games we counted up 32 dead goblins at the end. But they were not all on the table at the same time. Looking at minions I reckon 24 should cover most eventualities, but that assumes you do as we do, and take dead bodies off, leaving a marker in it's place.
Of course that means 24 orcs, 24 goblins, 24 skeletons, 24 lizardmen etc. Looks like I need to get more skeletons myself.