It varies. Essentially a spot beam antenna points at a small section of earth. But, the spot beam antenna can have multiple transponders transmitting using that antenna. In each Transponder frequency Dish has been putting up 7 (or even 8) channels.
So, it depends how many transponders can be used with a particular spot beam antenna as to how many channels can be available in a given area.
The way satellites have been designed in the past is that a set of frequencies are reserved for spot beams. Then they are used over and over again in different antenna. Adjacent spots need different frequencies or they interfere with each other. So, one may use 1 & 3, and the adjacent one 2 &4.
They are limited by power constraints on the satellite. So, even if more transponder frequencies are available for a particular spot beam, they may decide to use the power somewhere else.
Not to mention other issues like uplink site capacity issues (like E12 they are double uplinking at sites to save on having to build uplinks but this limits the number of TPs they can use).