It is impossible to say for sure whether Amanda ever did actually receive seventeen gentlemen callers in the course of one afternoon when she was young; they might all just be part of the idealized picture which she has  built up about her past, and it has become a memory that she constantly refers to (much to her son's annoyance).  It is unlikely that she would tell outright lies about her past, however, and it is probable that she did use to receive a large number of callers. Essentially, though, the seventeen gentleman callers are an enduring symbol of her popular and successful youth.