I'm struggling with this, too. The Prima e-guide shows 9 on the map, not including the 2 in Valammar:
1. Dead Ram Gorge (the "Dungeon" in the northwest, which also has a veilfire glyph)
2. In the cabin east of the landmark in the middle of the farms (Tyrdda's Lover); you end up here at the completion of Hinterland's Who Who.
3. The cave called Wolf Hollow (where you go to defeat the wolves to prove to Master Dennet you can keep the horses safe)
[this is possibly bugged and/or does not appear]
4. On the roof of the western guard tower in Grand Forest Villa (the mini-fortress to the southwest)
5. In Fort Connor (not really a cave, just a half-collapsed tower of sorts just northwest of center)
6. Inside the square cabin just east of Fort Connor (you'll have to do a bit of jumping over fallen planks to get this)
7. East of the Templar Camp (IIRC, this one is either actually in the templar camp, so you can pick it up as part of the related quest, or it's in one of two places west of Upper Lake Camp: the cabin where you find Blackwall, or the two-tent mini-camp just north of the wall by that cabin)
8. Directly northeast of the Crossroads, but you have to go up the tower at Redcliffe Road to get it (I think). This should also be almost directly south of a Bottle of Thedas.
9. In one of the towers in the mini-fort that leads to Winterwatch Cave (on the western side of the SE Hinterlands; there's a related quest in which you have to seal a fade rift to convince some religious people you're the real deal).
10-11. In Valammar: one in the Vaults and one in the big room where you find the second gear.
12. unknown
Any help with the last would be greatly appreciated (as would any corrections). It would really suck if this thing were glitched... (Unfortunately, I think it might be, as I've checked the 11 known places but have only 10 tiles registered.) It would also be very helpful if we could see which tiles were missing. The mosaic image under the menu (Collections > Mosaics) doesn't accurately reflect how many you have.
ETA: All 12 of the Archdemon tiles were found in the Western Approach, so I think it's restricted to the Hinterlands.
Also, I'm wondering if it's in that room that only
can open as part of an Inner Circle quest.