Depending on the tech level and planetological constraints of your conworld, not to mention any phlebotinum present, it might be harder or easier to get data that helps you set parameters in which to work. If your conworld is pretty 20-century-ish and has no significant magic, a planet pretty much the same as ours, and more or less similar culture and biology, then yeah, take RL data and play with that. However, if your conworld is, say, neolithic and set in a 2,5G planet that gets twice as much light as ours and spins around once every six hours then.... yeeeah, not so simple. Even harder if you have different biology, and practically impossible -or incredibly easy, depending- if there's any sort of magic whatsoever.
As a general rule for premodern settings without magic and crop yields similar to those on earth, take anywhere between 10 to 40 people per square kilometer of settled, reasonably irrigated arable land. This gives you roughly between 3 and 15 million people for someplace the size of Montana [I just had the wikipedia article for the state of Montana open, so that's my example, yeah. Looks like a pretty nice place, too, I should go there sometime before I die].
Do consider, keeping in with the example, that Montana has, in reality, like one million people living in it, modern agricultural methods and all. That's mainly because most of almost everywhere sucks for agriculture. Mountains, canyons, desert, cliffs, forests. Most of earth is either too dry, too rocky, too wet or too uneven to make for easy agriculture, and even then, a lot of ground just sucks. Depending on dominant biomes, just work out what percentage of your region is farmland [this depends on the total that *might* be farmland minus the pieces that no one has turned into farmland] and multiply that by 30 and you have a nice ceiling for the population of your region: however this question is less relevant than you think: most places, especially before the industrial revolution, are far less populated than they might be.
More towards more efficient farming and that number grows, and denser, richer in CO2 atmospheres will rise it as well. Less solar irradiation and longer nights will probably lower it, unless the local vegetation is adapted to them, and then it likely lowers it anyway. a denser air makes for easy gas exchange, as does CO2 richness: more nice gasses for plants to make shit happen with. Another relevant question pertains to soil richness: volcanic geologies produce richer soils, as do moderately complex ecosystems. The history of the region is also relevant: former jungles, for instance, tend to have shitty soils, and former deserts tend to have thin layers of soil over a large reservoir of sand, which sucks.
Do consider rainfall and temperature: a pinetree grows thrice as fast in Buenos Aires as it does here in Santiago, and both cities are practically the same distance from the equator, its just they get more moisture and far more heat, not to mention milder winters and flatter, richer soils, whereas Chile is mostly pebbles. okay, I'm exaggerating.
The point is, work out a ceiling and multiply it for the percentage of unused land in the area; Europe now has like 2% non used land, but a really large chunk of the americas is unsettled nature: hell, I took a 500 kilometer drive the other day and most of the roadside scenery was an endless series of shrub-covered hills, and this country has TEN times the population density of Montana. Modern day france, for instance, has 100 people per square click, and many countries have more than a thousand per square click. yeah, kilometers is clicks now, I'm in Vietnam, apparently.
Do keep in mind that the ceiling is by no means a guide: Uruguay, one of the most blessed countries in the planet arable-landwise, merely has 20 people per square click. that is a pretty reasonable density, as far as premodern countries go, as far as I know. Why, you ask? because they're cattle people, they use the land more for feeding cows than for growing stuff... that and they don't breed so much. Anyway, yeah, for premodern earth-likes, take a ceiling of about half a million people per New Jersey sized chunk to be the pretty much maximum amount.
Demography-wise, the more primitive, the more people live in the fields: or better yet: until the agricultural revolution, 90% of people live in small agricultural villages. or 80%, or something like that.
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