Apparently it is different for each person
A supernumerary nipple (also known as a third nipple, triple nipple, accessory nipple, polythelia or the related condition: polymastia) is an additional nipple occurring in mammals, including humans. Often mistaken for moles, supernumerary nipples are diagnosed at a rate of 1 in 18 humans.
Carrie Underwood during her American Idol audition admitted to have had a third nipple which looked like a mole.
Mark Wahlberg, actor and producer as well as Calvin Klein underwear model and former singer known as Marky Mark, has a third nipple. He famously spoke about it to the press before considering an operation to have it removed, but has since reneged, citing his third nipple as his "prized possession".