The number of nipples to buy is dependent on how often you want to clean them or replace them. Some people buy several packages of nipples because they might not last long with wear and cleaning. Others buy just one or two packs and keep up with the cleaning after each feeding instead of soaking them and then cleaning all at once. The age of the preemie only matters for the nipple size and the amount of milk or formula they need for the feedings. The number of nipples to purchase has nothing to do with the age or size of the child.