There are 14 jester spawns in the Black & White area (the only 14 in the zone). Two of them are in the water in the room and are easy to miss. They are each on a 24-minute respawn timer, and the masks have a 5% drop rate. Assuming my math is correct, this means you have a 51% chance of getting ONE mask (of either type) per clear, or a 30% chance of getting a specific mask per clear.
The mad jesters have about 85k in health and hit for 800+ (probably higher depending on mitigation).
The insane jesters have about 150k in health and hit for 950+
The Hidden Jester has about 174k in health, hits 1050+, and AE rampages.
The Hidden Jester drops two items from its loot table.