It seems erik_squires know very well room acoustic...
I recommend to listen to his advice...
His advice here will help greatly to perceive a natural timbre experience which is only possible by controlling also timing of the direct and indirect waves ...
Room acoustic cannot be figured out only with the metaphor of boucing rays anyway, a room is an enclosure of competing pressure zones also... Then like says our friend here the room must be treated globally...
Thanks
How many and how big??
Of course people will advise you to put them at 1st reflection points but honestly I find that alone this is never enough. You need to have a critical mass of room treatment before you get a noticeable benefit IMHO.
Spread it around!
My point is, if you don’t have control over the room decay you won’t notice first reflection point absorption. I’ve treated several rooms, and never have I heard a big improvement by treating first reflection points alone. Only once the room was better treated did those panels suddenly become worthwhile. We pay too much attention to first reflection points, IMHO. It’s not, alone, going to do much good.