Really depends on the type of build of the horse, for instance, a thoroughbred would not take as much weight as a heavyweight hunter or a heavy horse breed (shire type). I would have though most heavier breeds would take up to about 16 - 18 stone quite easily, maybe even 20 for the bigger ones, although I doubt many people weighing a lot more than that would be riding anyway!
Agree - it very much depends on the build of the indivdual horse - it also depends how the rider rides - some people ride heavier than others of the same weight - sounds nonsense, but I know what I mean!!
Using a weight tape - weigh your horse in kilograms. Times this number by 2.2 to convert into pounds. For example: 16.3hh weighing 600kg = 1320lbs divide this by 6 - a horse should only carry a sixth of its weight - 1320/6 = 220lbs. Divide this by 14 (convert into stones and pounds) = 15 stone 7 ish. It all depends on type, ie a shire/draught/clydesdale are bigger boned and probably more able than the likes of thoroughbred types as lankeela and Memarino mention.