LOR for Specializing

I currently work at a multi-speciality practice as a dental assistant, mainly assisting the endodontist of the practice. I will be a D1 at Penn this upcoming fall. Assuming I do well enough in dental school, I would like to specialize in either ortho or endo. About how many lor do you need to apply to specialty programs and who should write the lor? Would it be beneficial to ask the endodontist I assist for a lor in the future?

Thanks so much for your help!!