I could only find 7 Game Center games on Mountain Lion. Are there more places where Game Center games can be found, or is it just that more will be added along the way?
If you open the Game Center app, at the bottom of the Games tab there is a "Find Game Center Games" button:
Which goes to this page which has a list of Game Center games (there are currently only 8).
Also, some apps on the App Store will mention in their description that they have Game Center support. And you can see some games are featured with text saying they've been updated to support Game Center: