My single 12” Rythmik sub is in a 20’ x 25’ room and it is more than sufficient for us. It all comes down to what you want and, as it’s been mentioned, how many different locations in the room you are trying to get bass to. There is a big difference between a setup with multiple rows of seats and, like in my case, only two seats right next to each other. For us, the one sub located behind the couch, does what we want it to. The volume of bass desired is also a factor. The more you want, the bigger and/or more subs you need. Different strokes for different folks.