What is the capacity a horse can carry? Well, it depends very much upon the individual horse rather than one flat figure for capacity.
The average horse can carry 25% of their weight as capacity, and a well conformed, stout, and healthy horse may be able to carry and additional 5%-10% more comfortably. You'll want a vet to examine your potential purchase evaluate conformation. A horse that is conformed well and proportionally will be able to carry more weight without adverse affects on their body.
When looking for a weight carrying horse avoid breeds such a Thoroughbreds, Arabians, Saddlebreds, and other especially light horses. Instead, look for breeds such as the Foundation Quarter Horses, Stock Type crosses, Halflingers, Orlov Trotter, Bashkir Curly, Mustang, or stout Morgan. Aged 5-15. Remember, a larger horse doesn't always equal a larger carrying capacity. Many larger horses were bred to pull large amounts, but are actually less suited to carrying heavy loads than small stock type horses, bred to carry large cowboys.
Many riding stables cap clients weights at 200 or 225. This does not mean a person of that weight cannot ride at all, but a person of that weight cannot ride a the average lesson horse (an older, light breed) for an hour then expect the horse to complete several other lessons that day. That is the reason many riding stables have a maximum rider capacity posted.