Thank you for your question, it depends on babies age, younger babies especially breastfed babies will get through more shells as they tend to get the shell dirty often, whereas a older baby on solids will need less,
As a guide 2 shells (3 if using overnight) 6 days inserts (7 for use overnight), a doubler for naptime for heavy wetters and a overnight insert for night time
thank you if you have anymore questions or want to get in touch with your babies age etc I can give you a more personal guide.