Seriously, what possible reason could there be for us to be capped on having only [i]ONE[/i] Memento of [i]ANY[/i] kind in our inventory??? Another arbitrary, asinine cap that is unhelpful and unneeded.
You can have one of each in your inventory. They are also massive disappointments, ugly shaders, names only you can see when you inspect it, and a tracker for a specific activity as if that matters to anyone...
After I got my first one I kind of stopped caring about them. Just another bit of meaningless fluff they'll ignore and probably do away with in a years time.