I see that this is listed as a common drop.
The first time I did the Necromancer Test of Power, I believe I got the Silver Disc the first time I went up (this was over 10 years ago). THIS time however (helping a couple necro friends), we had to clear azaracks (yes, even off other isles since their pathing is messed up still) a total of 5 times. The first 4 times, they dropped platinum, copper and mithril discs most commonly. I saw one gold disc and maybe even a bronze disc iirc (didn't loot them). I do know we got them all because I had a ranger buddy tracking to make sure we did get them all hehe.
Finally the last time we cleared them, the second and third azarack we killed dropped our two silver discs we need. I would not call that all that common. Took over 24 hours of camping Sky to get them. I wouldn't call is "rare" either though. Just not common IMO.
If you need a Silver Disc, good luck! I hope it drops for you the first time necromancers!