One or two subwoofers?

It all depends on the circumstances. If the crossover frequency will be high(>100Hz) and/or the low pass crossover slopes small (<18 db/octave), stereo subwoofer are more necessary because they will operate into the area where the ear can localize the source of sound more easily. The best way to test this is to play music with only the subwoofer operating. If it can be localized by listening then stereo subwoofers are called. Using two or more subwoofers (stereo or not) can result in a smoother frequency response in the bass although this will require more thought and experimentation in the placement of the subwoofers in the room. A sound meter and test discs will greatly aid in properly setting up any subwoofer in any case. Good luck!

Agree with Rkeman, Crossover frequency should be determined and set at a point above which your main speakers bass no longer has flat frequency response. Small speakers require higher crossover point, this can cause image shift with only one sub unless it's placed between the main speakers.

Dave

Ideally your main speakers would be full range and not require an auxiliary SW. To achieve this, a SW should be adjacent to (underneath or behind) each of your main speakers, and become part of that speaker system. So this means that you need two SW for stereo, and three for multichannel (assuming that the rears can get along without extreme LF).

Apart from the spatial advantages of stereo LF, having several SW means that extreme cone excursions will not be necessary to achieve suitable volume, and that's good from the distortion point of view.

The single SW is an economy measure. One is better than none.