If you want to know how many people serve on a city's governing body, which document do you need to consult?

the city's home-rule charter

Explanation:

The home rule charter will provide all the information regarding the responsibilities that actually given to the government compared to the responsibilities that can be taken on by the local people.

The charter itself is not going to give specific numbers on the amount of people serving on a city's governing body.

But it can be used to consult or assist your research. You can find out all the responsibilities of your local government, and make a rough prediction on the number of people needed to run it.