The Collector's Problem

During the current spring promotion each box of Wheaties contains one poster of a famous woman athlete --- either Steffi Graf, Venus Williams, Chamique Holdsclaw, Michelle Kwan,  Karrie Webb, or Jackie Joyner Kersee..

You would like to collect a complete set of six posters.  How many boxes of Wheaties do you have to buy? 

           
Steffi Graf Chamique Holdsclaw Jackie Joyner Kersee Michelle Kwan Karrie Webb Venus Williams