According to University of Cincinnati's Claremont College, women do not have any extra ribs. Each human has the same 12 pairs of ribs, although an occasional exception does have one or two extra ribs.
University of Cincinnati's Claremont College states that the most likely origin of the myth of an extra rib in women comes from the Bible. The Genesis creation story states that the creator made the woman from the rib of the first man, implying that the man would have one less rib afterward. However, it never states how many ribs he had to begin with, nor how many she did when created, nor does it state that their descendants inherited their forefather's number of ribs.