Milk Line

Theoretically possible, but not likely. All mammals have milk lines, two parallel lines that extend from the arms/forelegs to he legs/hindlegs. Mammary glands and nipples can form anywhere along those lines. That is why primates have breasts up front, while large animals have udders in the back, and other animals such as pigs and dogs have nipples along the entire milk line. Extra nipples are fairly common in humans, but usually only one, or sometimes two (not like the picture above.) Check out this Wikipedia article.

Mark Wahlberg's Extra Nipple