Debby Herbenick looks at how men and women tend to answer the question differently:
Women tend to report lower numbers of sex partners than men, which, even accounting for same-sex couplings, makes no mathematical sense. One possible explanation is that women tend to tone down their sex numbers and men tend to exaggerate theirs—or at least to count more people as "sex partners" than women do. Other research shows that men and women estimate sex partners differently; that is, men tend to give rough guesses and women tend to tally their partners, listing them mentally by name, arriving perhaps at a more precise (and often lower) number.
The Dish 