This recipe says 20c will do an 18-24 pound bird. I never actually stuff the turkey, though, I bake the dressing in a separate casserole pan, which makes it easier to determine servings (and easier to make sure that the dressing is cooked safely).
Charlotte, midwife to some awesome women, wife to Jason, and no longer a mama to all boys S ('01), A ('03) S ('08) and L ('10).
THE BEST flavor for stuffing IME is to partially stuff the bird, make dressing on the side, then after the bird comes out take out the stuffing, mix with unbaked, and then finish in the oven while the bird rests. Seriously good!