I would say probably a bit more than 15 or 16 stone, dependent on the horse's fitness and the activity being asked of it.
To give a comparison -
My 15.2hh TB, who is pretty fine and slender had no problems carrying me at my heaviest of 15st on hacks of up to an hour. Some people can carry a deceptive amount of weight, and when I've told people how heavy I actually got after last winter, they were quite aghast and wouldn't have estimated me at over 12.5 stone.
Mostly it depends on the fitness and workload of the horse and the balance of the rider. A well balanced 12st rider won't weigh less than 12st, but an unbalanced rider of the same physical weight can be a heavier burden for the horse to carry. I daresay Salsa would have severely struggled had I asked him to carry me around a cross country course or on a long pleasure ride, but for around an hour's hacking or flatwork schooling, he was fine.