At some point, they build a second and possibly third city, but I'm still thinking that most of the planet is uninhabited. I need to have a rough sense of how many people are on the planet, how many are in the capital city, and how many are in each agricultural district (of which there are hundreds).
As a vague initial guess, I'm thinking 20,000 in the capital city, and another 500,000 in the surrounding districts. Maybe roughly 3000 / district?
Ideally, I'd like to have enough people in a district that it's a politically fairly important number, worth going to some effort to secure its votes. But a small enough number that the leader of the district could plausible have met most of the people in it.
Numbers are not my strong point!