If you google "10 ways to respect a photographer" you will find a number of tongue-in-cheek statements describing some of the things that professional photographers find irritating. This one in the image above is my personal favourite. The point is this - it is entirely normal to take multiple photographs at a wedding or studio session to compensate for those captures that go wrong. These errors will include mistakes by the photographer, poor subject position/expression, abnormal/unplanned movement, an unplanned change in lighting conditions, or many other miscellaneous reasons. If the photographer says he has a certain number of usable images, this is because he isn't prepared to release sub-standard work to you. Simple!