how much to buy for nb n s size diaper

I gave birth last mth and used 4 packs of nb before switching to size S. My baby was a premature so he's small in size but normal weight and now he's almost 5kg. Don't follow me or others cos our babies are all diff in size & weight. Buy 2 packs first bcos nb can use up to 10 diapers/day. Don't stock up too many cos they really grow very fast.