Newborns and infants will go through around 10 and 15 diapers per day depending on how much they poop or pee.
You'll be changing a lot of newborn and infant diapers but as the baby gets older they will use less diapers and you'll usually only need to change your older baby around every 2 to 3 hours.
You should check the child's diaper for wetness or for poop at least every 3 hours and after 3 hours they will most certainly need changed into a new diaper.
Toddlers will usually go through 5 diapers per day.