Limit to programming transponder keys

I have 2006 Sienna and I have successfully managed to program 2 transponder keys using already programed key. The programed keys were not cut accurately and would not work. Hence I need to program the keys again. Now car will not let me program any more keys :mad:, is there a limit how many keys that can be programmed. Is there a way to reset the computer and start all over again. Thanks

Ggohil
ggohil@yahoo.com