One of my girls has 10 and 2 nubbins (remember Chandler in Friends and his nubbin??)
The two nubbins look like little warts next to her real nipples and when she's nursing milk will come out of both nubbins. (lucky puppies who get on the nipples with nubbins next to them)
My mama girl and her two daughters each have 10 with the extra two on either side at the end. The extras are right next to the end two nipples. I never noticed if the extra two actually produce milk though...