I need districts but don't know how many

[Azélor]

According to Medieval Demographics Made Easy (I trust his research so that I don't have to do my own), 1 sq. mile of farmland can sustain 180 people. So you're looking at about 18,000 people tops. You may be able to push that to 25,000 based on your magical farmland, but I wouldn't go much higher than that.

Magic only has limits if you want limits.

Your estimation is for the total population of the area = rural + urban.
The urban population is generally 5-10% (15% in a very rich region).
So that give a city of more or less 2000 people.

Also, with 180 people per square mile, that's about twice the highest estimation for the population density of France before the black death.
You need a subtropical climate to sustain these numbers without magic.