Although there is no law that has established a limit on the number of times a woman can be a surrogate, the guidelines set forth by medical professionals indicate that a surrogate candidate cannot exceed a total number of 6 pregnancies, including prior pregnancies.
This means that, for example, a woman who have 5 children of her own, would only be allowed to have a 6th and final pregnancy. A woman who has already had 6 pregnancies won't qualify to become a surrogate, though.