Actually, you can have 255 cities! DoM figured out how to do it. You have to have restarts on, and at some point in your game, you must kill off a civ and have the respawned civ start at a location where it cannot build a city. Than you keep that AI settler trapped where it cannot build a city while you destroy the other civs and build your city count up to the 255 limit. As long as the AI settler exists, there is an active AI civ and the game goes on, alowing you the build that 255th city.