I think it probably depends on a) how much is in each bottle and b) how much he's eating from his meals. Every 9 month old will be different, so there are no strict rules. But if he isn't finishing his bottles, he's probably not hungry, which means he's had enough. You could try putting a bit less in some bottles (the ones he doesn't finish) if you're worried about wasting milk, rather than cutting out a feed entirely.