How many are there? Have two in the device but still no response.

3.

These are random loots in the chests right? I found two as well, checked every corner of the Cairn but can't find the third.

There's 3, and they aren't random, though they are in chests. I can't recall the exact locations, but it doesn't matter all that much. The reaper is a one-time spawn enemy with no good loot except 3 unfilled Black Soul Gems and an Iron Battleaxe.

There is a glitch, or at least there was for me. I had 3 crystals and activated the altar, and all 3 were removed. But 2 of them were in a stack and it said: Reaper crystal removed, Reamer crystal removed (2). The (2) only counted as 1 though. So there are only 2 crystals on the altar even though I had 3. I also searched all over the place for more, running in circles for 2 hours. I managed to find 9/10 Opus pages, and the Wrathman summon book, but no crystals.

Leave Cairn and go back after a few days. The crystals reset sometimes. Though if you can't interact with the atlar anymore, you're boned.