lindabean.......why are you down to 11 hens? You must have, at one time, had more. If they disappeared, I would work on what was the reason and fix weak fences, possible cover problems, and preditor proof everything.
As to when will you be able to stop buying your eggs......I would say, about 20 weeks after you get your white leghorns. If you loose any more hens and decide to start over with a few chicks, I would get whatever is your favorite breed and make sure you have a few leghorns, minorcas, silver-spangled hamburgs or anconas; thus insuring you a guarantee of eggs. Just make sure they know what a nesting box is for. The silver-spangled hamburgs are real good at hidding them.