This exact question seems to come up about 10 times every year. I'm not jumping you but folks please read the manual that FIRST spends a lot of time writing! This subject is well covered by rules R27, R45, and R46 as it is every year. If you don't understand the manual (which can be very confusing we all admit) please search CD before you post.
To answer the question: You are not allowed to use any motors other than those that came in the kit this year and in the quantity that came in the kit. You cannot purchase additional motors of like or different kind to use on the robot in addition to those that came in the kit. You can purchase additional motors as EXACT replacements (from IFI or Banebots) only to use as spares. You may purchase 2 more CIM motors and 1 more mini-bike motor to use in addition to those that you were shipped in the kit. The checklist only listed 1Fisher-Price motor but we all (should have) received 2 in the KoP and per team update #1 we may use 2. Additional servos identical to the ones in the kit are allowed.
By my count you can use on the robot at any time:
4 - CIM
2 - mini-bike
2 Fisher-Price
2 Globe
2 of the window motors with 1 left and 1 right
1 of the "other" window motors either left or right
2 Banebots motors (one supplied with the gear box, one without)
1 small Mabushi motor
As many of the servo motors as you can make use of
(I am going from memory here so if I missed any someone please add to the list.)
Unfortunately, our dear friend, Mr. Van-Door, passed away this past year and won't be here to help us out.
Please see rules R27, R45, and R46 in the manual and update number 1 for more information.