Horses have two nipples on their udders and tend to only have one foal. Most times I'm told twins don't make it and are often culled "in utero" to save the life of the mare.
When not in late pregnancy or nursing a foal mares teats as they are called are usually small and inconspicuous. It's still a good idea to pay attention to them since abnormalities caught early can be treated in case there is an infection or cancer. Also mares can rub their tails raw if their udder is dirty and itchy. One must take care and use a warm soft cloth to clean the area. If you are female just think about the sensitivity and if you are male, well....there are parts that are pretty comparable!!
Take care of your girls!! Regularly get her used to you handling her inner thigh and teats, start slowly and a little at a time; approach and retreat until she doesn't care about you messing about. My mare purses her lips like I'm getting an itch that's been killing her forever!! Just be careful since "Man that feels good!" is not to far away from "Ouch! Me no likie!!"