For may campaign I want to correctly place the cities in my kingdoms.

Is there a guideline how many cities could be in a certain region?

Lets asume its a middle-age-like culture and the kingdom is eg. 300 miles * 300 miles (500km^2).

I know that this is strongly dependent how fertile the kingdom is, lets asume thats also an average value.

Are there 10 or 100 cities, I have no idea for a correct number.

Thanks for help.