Work backwards:
- Choose the specific speaker that creates the tone you want - no matter the size or power handling
- Choose the amp power output you will need for your application - i.e. bedroom or gig?
- If the amp power output is more than a single speaker can handle, add another speaker until the power handling requirements are met
- If you like to blend speakers, say celestion blue and gold, then obviously two or more speakers are needed
One could also talk about the difference between speaker break up and amp break up. If the speakers are at their limit at max amp output, then you will hear the speakers also changing the tone and not just the amp. If you are not a fan of speaker break up or you want nice clean tones for effects, then rather add more speakers than required by the power handling criterion.
The point is that the number of speakers required should be more about power handling requirements than spatial effects of filling a room. The guitar is, after all, a mono instrument - adding speakers in mono does not have the same benefit as adding speakers in stereo - just look at the evolution of home theater systems... And besides, recordings of guitar amps are typically done with one mic facing one speaker, right?
Well that is how I would look at it!
