One or two subwoofers?

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.

I added one Linn AV-5150 (2 12" drivers) to my Linn 2 channel system. It was dialed at 48hz and placed in a cornner at the back of my room. The sub received signal from both channels but you could always tell the location of the sub. I got another AV-5150, set them in a stereo
configuration and placed each in between my listening chair. I love the way the sound now. The bass form my main
speakers (Keltiks) did became less boomy. I get the sub help cancel some of the waves.

On a 5.1 system the .1 is a special channel for low frequency effects. One sub will do but for music on a 2 channel system I prefer to use 2 subs. Hope this helps.
Hector

Invisibility and blending are a function of crossover point, slope, type of crossover (level matching & damping controls), number of subwoofers and (last but not least) matching tonality of subwoofer and woofer drivers. I have paid attention to all these and I get perfect blending and invisibility between 4" woofers and 10" stereo subwoofers using a modded Marchand electronic x-over.

Stereo subs is the way to go. However, it requires more effort in initial setup. I am using stereo Sunfire Signature subs with my MG 3.5 (I am also biamping with a Marchand XM126s) Blend is seamless. All musical parameters have been improved (caution: go easy on the voume control).
Depending on the frequency response of your main speakers, use the lowest x-over point possible.

I'm running three subs, one from my pre-pro's LFE out and two from an extra set of pre-outs. I use an outboard crossover for the stereo pair set at 35hz and run my mains full range. The third sub only comes into play for movies.

Dave