The number of channels on an amp doesn't limit it to the number of speakers you have connected, but impedance (ohms) and the frequency being sent to specific speakers does.
You can put as many speakers on a channel as long as the impedance doesn't fall fall below what the amp can handle ot the power handling of the subs isn't exceeded by more than 10%. Since subs cannot tolerate high freqencies, they too are usually driven by a seperate amp as well as the difference in power. Most systems will have higher powered amps for the subs than the other speakers.
An example, four subs and each sub is 100 watts RMS @ 4 ohm and the amp is 400 watts @ 4 ohm, you will have no problems with wiring all four subs to the amp like so http://i81.photobucket.com/albums/j230/sparky3489/SERIES-PARALLEL.jpg as this results in a 4 ohm load.
See my site for more info on subs, enclosures and other tips http://spkrbox1.spaces.live.com
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