Surprisingly not that many:( I've logged in 140hrs and haven't ran into that many. I have two masks. That's it. There's a total of 10. I have a feeling I will meet up with the legendary Krosis. I'm at the stage in colletcting the Scroll on the main quest and I think he'll come in very soon. Heard he's very tough to beat.
I have put down all 8 of those priests.
If you take all of their masks to labaryithian and get the wooden mask by the skeleton then something special can happen.
In the same room as the wooden mask you pick it up an put it on. (only works in that room)
This will transport you to the past were you can place all of your masks on their corresponding statue.
When all are in place the carved dragons head opens up and the final dragon priest mask KONARIIK (warlord) is revealed.
If your health is low the mask will activate and will heal you and your allies. Knock back (simelar to "fus ro dah") and light your enemies on fire. Did I mention that it has a chance to summon a dragon preist to slay your enemies with impunity.