So far, there are 5 missions (including the tutorial) and there is 1 mask hidden per level, with 6 masks total (so far.)

The Smiler Mask is unlocked by default, so there will be 5 additional masks for you to find.

Warehouse: Once you get to the firing range, climb over the table that has a shotgun & tec-9 on it; go to the very back wall, climb the box, then the shelving, and on a table in a hidden room with a flaming barrel will be the Birthday Girl hat

Farmhouse: Climb onto the roof of the main house and get into the attic without dropping into the 2nd floor - the Sloth Mask is near some boxes against the wall.

Sunset Motel: In the main parking lot of the Motel, there will be a red explosive barrel beside a window with 2 boards on it; blow the barrel up by shooting or hitting it with a melee weapon & climb through the window - the Armadillo Mask will be hanging on the wall.

Train Station: In the underground portion of the map, near the spot where you kill the Chef - it's sitting on a table/stove, you'll unlock the Scary Face mask.

Additional tip: You can unlock a secret weapon (the Machete) by killing the Chef, picking it up & finishing the mission with it in your inventory.

Suburbs: After killing Killswitch, there's a small pipe coming out of the overpass wall that you can crouch into and follow to find the Jack Rabbit mask.