i'd be surprised if he doesn't win more than 14 GS tbh.
IMO that kind of thinking is looking at what has happened and assuming that is what will happen going forward. It just doesn't work that way. It doesn't take into account Nadal's declining years, which officially started a couple months ago at age 26. It also doesn't take into account injuries, which he has had plenty of in the past and will likely have even more of in the future as he gets older.
I know some think 10 RG is practically auto, but reality is it's actually unlikely. It's easy to ask who can stop him, but that is overlooking how unlikely such a thing is in tennis. No one has won more than 7 of any slam ever. Nadal will almost invariably get 8 RG crowns and break that record. I think he's got a shot at 9 even, but 10 requires the stars to align. No injuries and no one in the next three years that finds a way to beat him. He isn't winning slams after 30. His game is too physical.