It depends if it is actually solid wood, or wood veneered.
A lot of the cheaper wicks doors are veneered. Doors are not my speciality, but I suspect a lot of these have a frame, then a chipboard/Laminate or dense core material inside the frame and then a veneer over that.
If it has such a construction, then cutting the door in it's width is cutting into the frame, it may be the core material isn't strong enough to support hinges, and so you can only cut away a bit of the frame, this seems likely in budget doors.
If it is actually solid wood planks, then I would view the 12mm as a guideline rather than a rule, you just need to look at the jointing methods (so you don't cut into a joint) and anywhere else you will need to cut into it (door handles) to see how reducing it may affect the door.
Hollow doors are easier, you can cut them over the recommendations, and then glue in extra battens to replace the ones you cut off.