Do I need 12 of these (2 per injector) or 6 (1 per injector)?
Yes, 2 per injector = 12. I would recommend changing them all out while you are in there (although others say you don't have to). I found a few that were brittle and cracked. If you relieve the fuel pressure, then take one injector out and leave it out, you can change all of them fairly easily without depositing too much fuel.
Also, this is a good time to change the valve cover gaskets (how-to here) if they are leaking. Keep in mind that you will need two gaskets (front and rear) along with 6 spark plug gaskets.
