There are 52 cards to collect (a whole deck). They are scattered throughout the mansion. You must collect them all for a hidden ending (in the Room of Old Habits). For the most part, you will be able to collect almost all of them by yourself, without any help. However, by the end of the game you might be missing a couple and have no clue where you haven't checked yet. It is highly suggested that you keep track of what you have already collected. Then, you can see which ones you need and reference the guide.
All 52 Cards are listed here with locations and Images as well as a handy checklist.
All solution credit to the actual author Tyger7
http://www.xblafans.com/sexy-brutale-52-card-locations-92725...