Izaro will always drop at least 1 Treasure Key.
Buffing Izaro with The Phases, will result in 2 extra Treasure Keys.
Argus drops 1 Treasure Key.
There is always 2 Treasure Keys inside curious lockboxes behind puzzles and gauntlets.
Darkshrines have a chance to give you 1 additional key.
The last 2 Treasure Keys have a chance of spawning together in one of the Treasure Chests at the end of The Labyrinth.