When we put our application together, we went with one as per the guide. Hope this helps! (link is below)
•Photos: Ask a photographer to provide you with a set of photos of yourself and each of your family members included in your application, whether they will be accompanying you or not. The required number of photos for each individual is indicated at Appendix A, under PHOTOS. Photos must comply with specifications given at Appendix B , Photo Specifications. Make sure you give a copy of these specifications to the photographer.
◦On the back of one photo (and only one) in each set, write the name and date of birth of the person appearing in the photo as well as the date the photo was taken.
◦Enclose each set of photos in separate envelopes. Write the family member’s name, date of birth and relationship to you on the corresponding envelope and close the envelope with a paper clip.
◦Photos must not be stapled, scratched, bent or bear any ink marks.
Note that the visa office may also require additional photos at a later date.
http://www.cic.gc.ca/english/information/applications/guides/3999E4.asp