It depends on your expectations, your mains & other factors. I will describe my 2 rooms and comment on each.
Room 1, 17.5’ x 24’, the ceiling is 9’ at the sides and 14’ in the center. There is an entry to a dining room and small hall that cannot be closed off. Subwoofer is a meager 10" 50 watt. Mains have two 7" drivers each. I was quite surprised with how well the sub did in this rooms volume. I considered this sub weak in every other room I placed it in before (and with every other pair of mains). I listen to TV and 2 channel music in this room. The bass frequency is not level everywhere in the room. More importantly, I found a location for the single sub that provides good response in the two primary sitting locations, the dinner table and the couch centered in front of the TV (I can survive 5 seconds of imbalance when entering & leaving the room). I might eventually build and try out four 12" DIY subs because I already have the drivers from a previous project. I don’t feel any urgency since bass is already adequate in this scenario.
Room 2, 15’ x 22’ x 9’ ceiling (much less volume than room 1). I can close all the doors. This is the home theater room with a 12" custom built sub tuned to 20Hz. My bass expectation is higher in this room since it is 7.1 with LF effects output. The sub is excursion limited at 150-300 watts. My expectation is not met in this room so I am currently building two cabinets for 18" drivers with 32mm excursion, they will be tuned to 14Hz and leveled with a mini DSP. In this room I expect to feel the bass below 20Hz whether I can hear it or not.
Hopefully you can place your expectations and situation relative to my two examples.
