This is a common question for parents getting started in cloth diapering. There isn't really a straight answer for this. How many diapers you need depends on three things:
1. How many children you are diapering
2. How old the children are
3. How often you want (or plan) to wash
The older your child, the less diapers you'll need each day. Newborns can go through 10-12 (some days we used 15!) diapers a day. Infants will use 8-10, while toddlers may only need 5-6.
Take notice of your child's patterns to see how often they soil a diaper.
Most peple find that washing every second to third day is manageable. This interval also keeps the diaper pail fresher. It can also help your diapers to wash better and stay fresher. Obviously, the more often you wash, the less diapers you'll need. Some people go up to a week between washes. You'll need a lot more diapers to do this, and may run into odor problems with your diapers.
Once you figure out a ballpark amount per child, add these together. Always allow for a few extra diapers in case you can't wash on time.
The number you'll need will vary on the factors discussed, but plan on having:
30-40 diapers for a newborn
20-30 for an infant
12-20 for a toddler
Don't go overboard-buy a minimum and add as you need to. You can always fill a temporary gap with disposables. If you're goal is to save money, you'll want to slowly add diapers to your stash until you find a comfortable number.