There is no hard limit on the number of photos that can be attached to a listing. Customers have successfully added or imported 20 or more photos added to a listing. Of course, there is an expectation that common sense rules apply. Attaching 50 photos to a listing, for example, could result in unexpected and undesirable behavior of some sort.
At present, mobile versions of schedule-a-showing pages display only the first 8 photos from the listing. This is the known behavior and will be improved upon in a future update.