It's the DMP who requests the number of photographs for the person to be examined. This is because in some jurisdictions, the VOs would request the additional photographs and affix them to form IMM 1017, then give it to the applicant to take to the medical examiner. Most DMPs would have their own 1017s and will notify you how many photographs are required. That is part of their duties as Designated medical practitioners, they have some rules to follow. Check the DMP handbook, Appendix C (http://www.cic.gc.ca/english/pdf/pub/dmp-handbook.pdf) if you're curious.
My wife was told to bring 4 photographs of herself to the appointment, who then placed them on the IMM 1017. A copy of that was handed to my wife to be included in the application bound for Mississauga.
So in short, your DMP will advise you how many photos are required and he'll ask you to bring them to the appointment.