There is no fixed maximum number of babies a horse can have. Mares can live 25–30 years, are seasonal breeders, and breeding frequency depends on owner management.
A mare could have a foal every year, but that is not ideal for her health. Frequent pregnancies can strain the body, and ethical breeders prioritize welfare over numbers. A healthy mare might have a dozen or more foals in her lifetime, but her health and well-being should come first.