How many police co-workers do I need?

I meant I have 16 sims in the police career, not at the station. So in theory the game should have more than enough sims to choose from when I sens my own cops to work. But it still uses townies and that's why I think that my police sims have the wrong rank or the wrong skills.

Edit: I rephrased my initial post, hopefully now my question makes more sense.