That depends entirely on the kind of accoustic challenges the room has.
However I can say that since you can't absorb more than 100 percent the benefit per thickness drops off after four inches.
You most likely will have to buy a whole bale (24 inches total thickness). What I do is buy 24 one-inch thick panels and then experiment. For example you might have four four-inch panels in corners as bass traps and four two-inch panels offset on the parallel walls to control slap echoes.