Depends on age, bone, fitness of horse etc and abilty of rider........ but my share 17hh warmblood (good bone, quite fit, ) carries 15.5 stone of me (average but well balanced)just fine
Its thought now that some "ID's" have up to 1/4 TB blood in them. Its how the breed is evolving with those enterprising irishmen.
When i was at badminton me and mum saw a huntsman who must of been well over 20stone on a 16.3ish traditional ID, horse did look pretty comfortable and the bloke was sat like a sack of spuds.
Like other has said depends upon the build of the horse. My horse is 16.1hh but is quite fine boned for an ID. I've let someone who was 14 stone have a go on her and I could she my mare struggled slightly. But I've seen chunkier ID's carrying alot more and they seemed ok.