The optimal number of subwoofers depends on your room and needs. Let's break it down below.
Bigger Is Better?
When it comes to subwoofers, which are low-pressure frequency devices, there is no replacement for displacement. A larger enclosure makes everything a subwoofer is trying to do easier. Build quality and design being equal, translates to higher output, greater pressurization, lower distortion, and less stress on the driver(s). But do you have the space for a larger subwoofer? And for those that have the space, do you really want to deal with the weight of a mini-fridge? But that is not your only concern.
The Enemy - Room Modes
Room modes are a collection of frequencies that exist in a room caused by a sound source like a subwoofer. A room mode can cause both peaks and dips in frequency response. When two or more waves meet with reflections off the walls, floor, and ceiling and are in phase with each other at a specific frequency, you will have a peak in response. When they meet and are out of phase with each other, they cancel and you end up with a dip or null in response. These "modal resonances" store energy and decay, causing audible problems such as "one-note bass" and "boominess," even robbing mid and high frequencies of clarity. On the opposite side, you can have severe cancellations that rob you of "punch" or "chest-slam".
Teamwork Makes the Dream Work
Two subwoofers, ideally placed, have a significant impact on room modes. The positive results are easily audible and measurable as well. The frequency response is much more even and balanced. Furthermore, the two subwoofers will work in conjunction to boost the overall output and increase pressurization. Together they achieve the same acoustic results at lower distortion because each driver only has to work half as hard.
See Where This Is Going?
It may be obvious by now, but we are big fans of dual subwoofers.