I have a super large washing machine
i can wash 30ish nappies + wipes / wraps / boosters etc
how many nappies you need depends on how often you wash, how many fit in your machine and how quickly you can get them dry.
many of mine dry overnight on the airer.
bamboo and hemp take longer to dry.
Im a cloth nappy addict and probably have about 200 in my collection, however that is an insane amount and no one (not even triplets) needs that many!
see how many you can fit in you machine and then have a few extra as spare while they dry..... there is no point at all having more than you can wash + use while the clean ones are drying
(I have 2 buckets and wash every 4 days, I wouldnt wanna leave longer than that between washing; i give them a rinse first (with extra rinse) then wash at cotton 60 intensive and extra rinse...... my nappies go round my machine for 4 hours 13 mins, and come out super clean!)
If I washed every other day (like most people i know seem to) I would probably only wash at 40.
In a few months when number 2 arrives and i have 2 in cloth, i'll be washing every other day i guess.