Applicants for Post-Doctoral residencies should have a good idea after NBDE Part I. You usually apply to Post-Doctoral residencies with PASS and Match programs in the summer between your 3rd and 4th year and you find out if you're matched by December through January of your 4th year before you graduate. NBDE Part II you do not even take until the end of your 4th year so it wouldn't come in to play as a decision factor AS MUCH.
About people wanting to specialize....yes....it's always a good question! We always hear and see applicants and first year dental students wanting to specialize UNTIL they get hit with 15 final exams in a row at the end of their first semester first year of dental school.
Specialty residencies/programs are moving in a direction where not only they want to see the academic numbers (class rank, cumulative GPA, and NBDE scores), but they also want to sense and document an early motivation from their applicants for their specialty.
Believe me, I've heard many times from Chairs of specialty departments at NOVA that they've turn down outstanding applicants (1st in their classes, >90 on boards) because they did not sense that "motivation" for their specialty. These applicants simply applied to Post-Doctoral residencies because it was their fourth year and they thought "heck, why not? I have the numbers!"
Keeping your options open is fine, but really do your research and homework on the specialty that may interest you. For example, do not wait until your second year of dental school to find out about Endodontics because that's your first Endodontic course, research Endodontics now and check out the Endodontic association web sites as a predent or a first-year!
Another example, I'm aiming to be an Oral & Maxillofacial Surgeon (OMS), if I waited (being passive) until I receive OMS exposure, then I wouldn't even see OMS procedures until my 3rd year of dental school because that's when OMS is integrated in the dental curriculi. Instead, I researched on the specialty as a Pre-dent by shadowing OMS surgeons, OMS departments at various dental schools, did OMS research, interacting with OMS faculty and now as a D-1, in addition to all that, I'm also exploring an OMS externship this upcoming summer which traditionally isn't even available for D-2s!
Be "ACTIVE" and not "PASSIVE" in the role of an applicant whether it's for dental schools or Post-doctoral specialty residencies.
You ask how many people do actually specialize?
I say 100% of those that are motivated from the start and active pursuing their goal to specialize, specializes!
As my high school football coach used to say to us, "WALK THE TALK!"