TheoTown

Do they add over time?
Do they calculate by population or anything else?

I think it has more to do with responding to emergencies and suggestions in a timely manner than it does population.

The growth of the city affects it too

@Lobby Is there a way to calculate it?

There is because the game has to do such calculations, however the function is quite long and has to do a lot of complicated math. To put simply, make sure your inhabitants always reach their demands and are happy.