It really depends on the student, but 5-7 is a good range. When researching possible schools, you may be interested in many more than that, but by the time it comes to apply you should be able to narrow the list. More than likely, you'll know the group of schools you’re most interested in attending.
On the flip side, even if you are dead set on attending your favorite school, it's a good idea to apply to more. There's always the possibility that you won't be offered admission. Then, if you are admitted, you will appreciate having a choice and not being limited. Your second-choice school may offer you a better financial aid package or you'll just change your mind. Bottom line, keep your options open but don't spend a fortune on application fees!