It's not limited in the game, you CAN build 6 cities, but in your mind, you should think 'can't build more cities ... no more cities ... no more cities... 
CivAssist II is a utility to take tiring micromanagement out of the game, and it does a dandy job of it! It can be found here, along with more info about the utility.

CivAssist II is a utility to take tiring micromanagement out of the game, and it does a dandy job of it! It can be found here, along with more info about the utility.