First of all you need to determine how often you will be doing laundry. With that in mind, here are some reasonable estimates of how many diapers you will need:
Newborn to 4 months - 20 - 24 diapers
Infant (4 to 10 months) - 16 - 20 diapers
Toddler (10 months to potty training) - 12 - 16 diapers

Please note quantities are based on an average sized baby and if you wash every other day. More diapers will be needed if you go longer between washings.

Please feel free to email if you have further questions.