i believe i found your answer. i found this from searching for 'city limit' in the conquest's editor help files. there's a file called 'What's new in this version of Civ3ConquestsEdit?' and if you go down to version 1.33 it says the following:
"Implemented city placement. You can now add, delete, and edit cities. When a city is placed, a unique name is automatically generated for it. City art will match the era of the player it is assigned to. (Note: There is a limit of 512 cities per map.)....."