It depends on the diaper, too.
When we're out in the world, my girl likes to wear as discreetly as possible if she wears, and the choice of diaper is usually Tena Lady Pants Discreet. When they are even a little wet, they require an immediate change, and she can go through a lot of them.
When discretion is not an issue, so she is comfortable in a more bulky diaper, she goes through a lot less of them.
So, I guess, 1-8(!) a day.