[Hakko]

Not an expert on medieval populations, but 10,000 is a lot of people for, say, a 12th century English county, unless it contained a large city. Wilderness regions like, say, the Auvergne in France, could easily have a couple thousand folks scattered over hundreds of square miles. OTOH, China had many cities of over 100,000 at that time, and a couple over 1,000,000.

My handwave is to accept that individual units of normal troops are actually company-sized (i.e., 100 men). Then large scale battles of 200+ units might be armies of 20,000, which is about right for the biggest medieval land battles. Magical units are what they are, of course. But it's easier to imagine that dispossessed spirits are company-sized, while a Lord of Hell is just one bad mutha----a.