There are a lot of questions on this site about historical population levels and densities, and they have good answers about taking into how hard it was to survive in the past, and how inefficient many practices were.
But I’m not asking about medieval Europe or China, this society has a lot more knowledge and resources at its disposal than pretty much anywhere had prior to the industrial revolution, but it still doesn’t have even the dream of a steam engine.
•Land area is roughly the size of the Iberian Peninsula and at the 40th parallel
•The region is very well watered with predictable rains and several navigable rivers coming from a major mountain range to the west.
•Soil is highly productive due to ancient volcanism.
•This region has a super crop that is extremely productive and nutritious, allowing for a well nourished populace.
•Crop rotation is a well known science to the population, with a program comparable to the Four Course System as opposed to the much more inefficient three field rotation. Biochar is also used occasionally
•Megapodes are raised for meat and eggs rather than chickens, leading to much greater quantities of meat and eggs compared to chickens.
•A large rodent speciesis raised as another source of meat. It reproduces much faster than cows or pigs.
•Seed drills, the horse collar, and mouldboard plows are used.
•A network of roads comparable to those of the Roman Empire connects all of the major population centers.
•The centralized government maintains storehouses similar to those used by the Inca in order to soften the blow of crop failures.
•The major rivers are all connected with canals.
•Aqueducts serve clean water to the cities
•Warfare has been avoided for several generations due to a powerful but isolationist government.
•The germ theory of disease is widely known and accepted due to an enigmatic prophet in their religion. Strong alcohol is used for hand washing rituals.
•Sanitation is likewise common practice, with sewage and trash disposed of carefully and water is often boiled.
•Healers here laugh at the theory of the Four Humors, and actually wash their hands before delivering babies and stitching wounds.
So with these factors in mind, how many millions of people could live in this region, or if that’s too broad would it be feasible for such a population to exceed 50 million?
Edit: How about 20 million instead?