Here is the full list of all 12 Inmates achievements.
Meet Ben for the first time
Find the half of the photo with Sarah on it
Solve the "Time Trap" puzzle
Find the inmates list
The nightmare begins
Pass the first subconscious level
Pass the second subconscious level
Pass the third subconscious level
Get discovered by the pop up creature
Solve the "Sound Trap" puzzle
Meet Ben for the final time
Complete the Game