I generally use 3 diapers a day for each child. I check them after snack, after lunch, and after nap. If they are really wet or poop in between these times I change them, but then they generally won't need to be changed at our diapering time. I think 4 a day is a good guideline, it allows you to have extras on hand. I require parents to leave a supply and I will let them know when I need more. I've never had anyone argue.
Now my first time mom sister in law just told me that they will only be changing their new born every three hours on a schedule. "Those diapers can hold a lot and are expensive." She said to me. She'll learn soon enough that there are occasions that the diaper will get too full and leak and stink and cause a rash.