Your controllers are most likely capped to control six axis, but you can confirm with Fanuc with your robot's information.
I know you can control more than one robot with the same controller and pendant, but that requires you to purchase and option and load it to the controller and also to purchase a card that is capable of more axis and replace the one inside your controller. As long as you are staying within the same family of robots, if you try to go from a 100i to say a SCARA.. you will need a additional controller