If you plan to BF, you *shouldn't* ever need the 9-oz bottles. Your breastmilk will change as your LO gets older, so in theory they should never need to take larger quantities of BM. Now, I've heard of some BF mamas giving 7-oz bottles, but IME and from what I've read, you shouldn't ever have to give a bottle bigger than 4 or 5 ounces.