I agree with dr-benj. Volunteering, as opposed to observing/shadowing, is above and beyond the expected. If you have volunteer experience, that is a major plus on your application. But I have not come across a single school that REQUIRES VOLUNTEER experience.
On the other hand, if the OP is referring to observation/shadow hours, I think it is an absolute must to have at the very least 30 hours time. That is not even a full week in the life of a dentist. You need at least this much time to even catch a small glympse of what a dentist does. For me, I did not even decide I wanted to go into dentistry until after I had logged many hours beside several dentists. Sure, what I had heard got me into the clinics to check things out, but it was what I saw that intrigued me, not what I had heard about dentistry.
Bottom line: expose yourself to the field as much as possible. But don't do it because it looks good to adcoms, but do it so you can gain information for yourself.