For Leah I have about 12-14 that I am using regularly, and Joel has about 10. (Leah only has more as they are the bigger size that they both may be in at the same time soon so if I buy any more I get the bigger size)
I wash every night, that is baby/kids clothes and all the nappies used that day, I dry bucket them but in the heat I prefer to wash more often, so there is usually half of Joels naps in the wash at any one time, sometimes more if he has a pooey day
, Leah normally only goes through 3 a day, but we are starting toilet training so if she wants to sit on the toilet then she also wants a clean nappy afterwards. ( I don't have a dryer but find esp at this time of year they dry quickly, if they are still damp then I put them in the hot water cupboard overnight and they are good to go)
Most people recommend at least 12 nappies, just depends on how often you are wanting to wash and if you are using some disposables like us.