Since the new pet slots, (20 total) this feature is only for Premium members, you will not be able to keep the 11+ slots if you cancel.
Once your subscription expires, you must abandon or transfer one of your seven Neopets to go back down to ten. You can select which Neopet this is – it does not have to be the one in the 11+ slots.
If you choose to abandon it, we will hold that Neopet for 90 days before releasing it into the Neopian Pound. (We don't want to put it in there immediately, just in case you simply had billing issues or decide to come back to Premium right away, etc.) After 90 days, it goes into the Pound to be adopted by a new owner.