How many you need would depend on how old your baby is, how often you wash and whether you are exclusively using prefolds or not. I have a 17 month old and wash every other day. Prefolds account for about 70 % of our stash. I have 18 prefolds and 7 covers but I always have clean ones left when I do a wash. For an older baby, I think you could probably get away with 12 prefolds and 3 covers if you are washing 3 times a week. For a younger baby (especially a BF baby) you would need more as they need to be changed more often and runny poo tends to get on the covers meaning they can't just be aired and re-used.