Ohh right, well I thought that the ribs were important regardind the size of the saddle as they function as a means of support, but i guess you’re right
The size of the horse will affect the size of the saddle, regardless of how many pairs of ribs it has. A full-size quarter horse will be able to handle a much larger saddle than a pony, and a draft will be able to manage an even bigger one.
And does that affect the size of the saddle??
Arabians sometimes have one less vertebrae and one less set of ribs than other horses, but they're the only breed I know of like that :)