There is currently no automatic way to make the page counter skip certain pages. There are various manual methods, the simplest being to use counter-reset to set the page counter on the tearout element:
counter-reset: page 12 /* tearout is really on page 13 */
We could make this process easier if we supported counter-increment for the page counter, then you could just do this:
counter-increment: page -1 /* step backwards one page */
However, this is not supported in Prince 5.1.
The other low-tech solution would be to place all the tearout pages at the end (or beginning) of the document, but that might not be convenient if it involves having to shuffle the pages around later!