When the game is generating treasure, it counts how many unique artifacts you have found (you can use the mm8leveleditor to see which ones are marked as found). If there are more than 13, it won't randomly generate a new artifact.The 13 isn't the limit on how many random ones you can get -- it's the limit on how many artifacts per game you can know about.
However, artifacts are also generated when a map resets into chests and other special places. So if you haven't "found" Excalibur in the rock yet, then it's not counted toward the total. That's because the save game only indicates a yes/no for each artifact rather than a total count of artifacts.
So, depending on how you play, you can get between 13 and 18 (13 + 5) artifacts.
13 are guaranteed to be unique, but if the random generator randomly generates any of the 5 pre-placed artifacts, it counts against your limit.
Why 13? Just an arbitrary limit the programmers chose. You can edit the exe to change it. Or you can use the MM8LE to go in and "unfind" artifacts so the game will start generating them again.
