Horses have pretty long backs - so, you would think they have a lot of vertebrae. Actually, a normal horse has 18 thoracic vertebrae (the ones with ribs attached), five of which form the withers, and six lumbar vertebrae. Arabians often have only 17 thoracic and five lumbar vertebrae.
Humans, in contrast, have 12 thoracic and five lumbar. This is not counting the neck, of course. Of course, this means that horse vertebrae are quite a bit larger than ours.