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.