How crowded is your town?
Every dentist in your area is in the federal register, so the size of the field is a fact, not a guess. Type your town and see how many practices you are competing with, how many of them keep their listing current, and how many are invisible at nine at night.
Have a look at a busy town first
These are the six with the most practices in the country. Yours will be quieter, which is the point: the smaller the field, the further one claimed listing gets you.
Why this matters more than a ranking report
Most marketing pitches open with where you rank. Ranking is downstream of two duller things: how many practices are competing for the same searches, and how much of the basic information about each of them a patient can actually find. A town with forty practices where six publish their hours is a very different problem from a town with six practices where all of them do.
Counted from the National Provider Identifier registry and from opening hours and websites published openly to OpenStreetMap. A registered practice is not always a trading one, and one address can hold several registrations.