How long did baby last in newborn size diapers? I am looking at stocking up - how many newborn size should I buy?

I only bought 1 packet of newborn size and proceeded to get S size ones as most S size can be used on newborns. Furthermore, newborn size diapers are more expensive than S size ones. You can also consider buying them 1 pack at a time if you prefers newborn size ones as baby outgrow them pretty fast. It will be wasteful if you buy too much and you can't change them to bigger size.