Fantasy author looking for tips on army sizes
Hey there. I'll try to keep this as light as possible. I'm a fantasy author working on a novel that's set in North America. It takes place in the future after the world has essentially become a "wildlife preserve", where humans have retroactively recreated natural landscapes (they removed the cities, infrastructure, basically anything artificial, etc.). However, a small population of humans stayed back on earth to live primitively. Eventually these humans began re-building a new society, starting from simple tribes and moving into a pseudo-medieval esque civilization (it's really closer to the Dark Ages, possibly even antiquity). They had no records or knowledge of the previous civilizations, so their progression would be natural and "fresh". These particular civilizations are centered around the eastern coast, with the major population centers being roughly around Pennsylvania, Ohio, New York State, Kentucky, South and North Carolina, Tennessee, and Georgia.
Kentucky and North Carolina, in particular, would theoretically be the most densely populated, as well as areas along the Ohio river. The only reason I really want to know about populations is for references to army sizes. My question is simply what you guys think the best way to go about estimating general army sizes would be in this particular context? There's "magic" (actually a form of nanotech remaining from the previous human societies), but it's rare and very minimal, mostly relegated to prophecies and telepathy and such, so it shouldn't affect the civilizations all that much in this regard (the biggest use of "magic" -- even though it's technically science-fiction -- is through an advanced, almost god-like transhuman entity, who has subtly pushed the civilizations towards advancement, as otherwise there's no guarantee that human societies living in North America would have ever progressed to a point comparable to ancient Rome or Greece).
I'm not how much the actual cultures matter, but if so, they're very, very roughly like a mixture between ancient China and medieval europe, with lots of fictional elements. They're supposed to be brand new cultures, so the similarities to our historical ones are fairly minimal.
I realize that this is a really odd sort of situation, and it's difficult especially because I like to make my fantasy worlds as realistic as possible (I'm also omitting an enormous amount of information here in order to keep this post short, so the whole thing probably sounds kind of unrealistic, but trust me when I say I've put a lot of effort into making the situation as believable as possible). I'm willing to make some concessions in this particular case, but I'd still like it to be relatively grounded in reality.
Thanks for any and all tips! If there's some other relevant information that would I should add, let me know. Like I said I've done a ton of world-building, it's just hard to know what to include here without going overboard.