Doesn't matter how many nipples, or mammary glands, your bitch has, keep an eye on her and run your hand under her daily and you will notice if there's anything wrong.
If you notice any hardening of the mammary gland, strip some milk out (you can use a towel soaked in hot water as a compress) and put one of the bigger pups on the teat. Antibiotics wouldn't go amiss either.