Daily DevotionFirst off if you get 95 devotion you can get 1 skeleton key and so roughly speaking for 1 month which is 30 days you can get 30 keys from this one:
Daily Check InThen from the daily check in rewards you get rewarded 4 times:
1 + 2 + 5 + 10 = 18 more keys:
DungeonsThen you got the blitzing of solo campaigns so let's say you do 8 per day out of which you get 3 keys then this will be 3 times 30 days so about 90 keys, but can be a lot more also.
Guild AltarThen you can also get skeleton keys from the guild altar, but it is a little bit unpredictable so it is not possible for me to guess how many. I suppose it could even be zero some month or a bunch in another.
(made this pic myself)