I have my horses on my own property. We have a herd of 4 in the pasture and two boxes and two yards, so in total our place could carry 8 if we were stabling/yarding.
Where I work, 18 is about the most for horses I actually feed, but then there are the horses in the big front pasture that don't get hay at all, and that pasture has a capacity of about 8... but it never has that many put in it because it's the "spelling" pasture. My boss puts her big gelding and her young TB mare in there, plus the two babies, a boarder's horse, and occasionally one of her other horses if someone is sore or just grumpy. Most I've seen in there was 7. So in total my boss's place can hold about 26 horses. More if she's stabling full time.