Hi there,
I don't have the stats on hand, but I can answer your other question:
Dentists can start practicing as soon as they pass their licensing board exam and graduate from dental school with their DDS/DMD degree. Residency is optional.
It is a good idea to do some sort of residency however; it's a good way to sharpen clinical skills before entering private practice, and learn some marketable skills not taught in detail during the doctoral program like implant surgery.
The newly-graduated dentist can also apply to and enter specialty-training for disciplines like ortho, endo, prostho, perio, pedo, oral surgery, etc. The length of those programs vary.
HTH!