120 film produces 12 exposures in cameras that (nominally) shoot 6x6 images, such as your Hasselblad. Other cameras shoot different formats; a camera that shoots 645 format for example will get 16 images out of a roll of 120 film, and a camera that shoots 617 format will get only 4 images out of the roll. Your camera is not wasting any of the roll of film – at least, there's nothing you can do about it.
220 film used to be produced. It was essentially twice as long as 120 film, but fit around the same spool by dispensing with most of the backing paper. It is no longer produced, though might still be available on eBay, etc. It would have allowed you to make 24 6x6 images from a roll of 220 film, but it required a different A24 film back. Your current A12 back is only compatible with 120 film.