A mono amp can power as many subs as you can throw at it as long as it has the power at the right impedance (OHM).

You can use a mono amp to power a pair of subs. You should do some research online and get familiar with bridging amps, impedance, and SVC v. DVD. A good place to start is actually JL's site, http://www.jlaudio.com/tutorials/index.html

Then come back with more questions.