Eddy
No set answer...
Photos: put in as many as you need to demonstrate the genuine relationship (photos with eachother's friends and family, photos on important days like holidays/birthdays, photos of traveling together, etc). You don't need a million photos of the two of you together just ones that demonstrate your life together and show the passage of time from the beginning of the relationship to present. If you're not sure, rather include some extras. I sent in around 100 photos covering three years of relationship.
Letters: The more personal the letters are the better. Make sure they include the person's relationship to you (friend, family, etc), how long they've known you and how well, when they first knew of the relationship, their knowledge of the relationship progressing, their knowledge of your plans to immigrate to Canada. As I said, the more personal the letter, the more genuine it will sound. If you just write a form letter and send it to people it will sound dishonest. Also it is a really good idea to include at least one letter from each side (one person that knows the sponsor, one that knows the applicant). AND, if the applicant is from a poorer country it is good to include extra letters from the sponsor's side as it will help demonstrate that they don't feel the sponsor is being taken advantage of for the purpose of a visa.