hmm. What I did was spend about a week of time with several dentists. Some were more sociable and willing to teach than others. I found a few that I liked that worked in one practice, and I stuck with it. It helped that there were/are a few GP's and specialists in their office, so it was good exposure.
Just my advice, but if you are seeking a letter of recommendation from a dentist you are shadowing, you should get to know them and their practice, and that takes time. With time as well, they taught me the basics of being a dental assistant, which is a good way of learning other stuff, such as how to tie a few different suture knots to close up a flap surgery. If all you are looking for is their john hancock on a job shadowing form though, 20 hours alone would probably be enough for that.