You aren't going to have peace of mind until you confront him so by all means do so as soon as possible. You can do it in a discrete, non threatening way....let him know you are just curious about his very large collection.
Many men have a few files of pictures of naked ladies on their computer. It's real easy to visit message boards and websites where they are posted, right click and save them to a file. My guess is that most men don't even ever look at them again because, frankly, they get pretty boring after you've seen them once...but some men just like saving them the same as other people download music they never listen to because it's free.
Yes, this could be something serious...or could lead to a serious addiction. But it could also be quite harmless. A good talk with get it all out in the open. If he's up front and candid with you, you'll have the information to make up your mind about what you want to do. Meanwhile, if you want to go to
www.google.com and insert "pornography addiction" in the search field you'll find dozens of sites that will give you additional insights into the problem and its treatment.
Good luck.