The least amount is best. Surrounding the area adjacent to the speakers with damping has not been beneficial in my experience. Read the thread by Michael Geen on tuning the room.
I purchased a new summer home that has a large great room and setup some old B&W speakers that never sounded that great in my current winter house. The room is fairly lively, but man do those old speakers sound like they never did before. There are no room treatments.
Returning to my winter home for a few days in which extensive room treatments are deployed I’ve decided the room is indeed too dead. I removed one tube trap and one 4x8x4 bass trap, added one 2x2 diffuser on the back wall where the big bass trap resided and sure enough the system sounds better.
I would surmise that like most things, moderation is best.