How many of these can be acquired? I want to enchant a few for lulz like one with fort lockpocking and still be able to keep an unenchanted one.

I found 4 during my escape from the Markarth mines. There may be more to be had if you make a habit of thouroughly searching each hold's dungeon, or that's where I'd expect them to be found.

For anyone else wanting to know, I did the quest that gets you locked in up in the mine and i got 8 shivs. Dunno about them being everywhere. I just like the fact that they have the same model as the piece in lockpicking. Thus my wish to enchant with fort lockpick.