If you go to foodnetwork.com. They have a turkey calculator on there. It'll tell you how many lb.s , people it'll feed and cooking time.
I don't put my turkey breast in a slow cooker. I like the skin crispy. I just rub some olive oil on it and season/inject it (whatever kind of seasoning that you like). 325 for about 15-20 min. lb. I do put a thermometer in it and take it out of the oven when temp. reaches 170-175 and set it aside and let it finish cooking out of the oven while I finish fixing everything else. Don't wait for that pop up thing or your breast/turkey will be dry.