How many ponies can a horse have?

I think you may be confusing the term "pony" with the term for a baby horse which is a "foal". A pony is actually an entirely different subspecies of equine that is typically smaller than a horse.

The gestation length for a horse is 11months, so mathematically a horse can give birth once a year after they have reached puberty which is around 2 years old. They usually are too old to carry a foal when they have reached the age of 20. So, mathematically a horse could have around 18 foals during her lifetime. However, most mares that are used only for breeding have around 10 - 15 foals during their life.