Each city has a fixed amount of slots. Minimum is one, max is five.

Capitals ('Cept for Berlin) all have five slots. They are the Government building, leisure building, barracks, admiralty, and cannon maker
Large cities ('Cept Koenigsberg) have four slots. They take away the admiralty.
Cities have 3 slots. I think they take away the leisure building
Minor cities have 2 slots, they take away the cannon maker
Small cities have one. It is simply the government building.
And all cities have walls.


As far as I know, you cannot upgrade slots, only the buildings that come in them.