Home theater: subwoofer quantity versus room size

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There is not only rules of thumb,but calculations if your really keen!
For a start you can keen it simple and consider
Sensitivity,Power(RMS),and SPL required at 1metre,this ignores the room
at the moment. For more calculations there a paper at jblpro.com

Location is just as important! A corner location for a subwoofer
results in a higher SPL but room modes are excited
maximally(theoretically)

The theory is that having more subwoofers , located in the room at the
right positions results in a more flatter frequency response since each
unit exites the room resonances with less energy(if I remember
correctly)

Note that one subwoofer is hard enough to set up - let alone 4. I would
get one,set that one up correctly,and then if im feeling rich get
another. Distortion will be reduced aswell by buying 2 units and
keeping SPL the same.

Generalised sweeping statement :
In regular sized rooms, with typical amplifiers available,and with
typical listening preferences a 10" or 12" of a litre or so
displacement typically is enough.

Regards
Mike.e