Newborns go through 10-12 diaper changes per day and older babies and toddlers (aprox. 6+ months) use 6-8 diapers per day.
Most Moms are most comfortable with washing every second day, but you can get by with fewer diapers and wash every day if you like or add more to your diaper stash and wash every 3 days. You don’t want to go longer than 3 days between washes.
Here’s a handy way to figure out how many diapers will work for you (click to enlarge):
In addition to the suggestion above, it’s handy to have 2-3 fitted diapers with covers in your stash for naps or overnight. This is especially useful for older babies and toddlers.