If your sub can handle all the power from the amp then I say one sub is fine! Only advantage I can think of haveing an extra sub is one sub not being able to handle all the power, or for looks. But personally one sub seems fine to me!
Loud music is good... Loud music that is clear is better
2002 GMC Jimmy
hu - pioneer primer DEH-P630
Sub amp-JLaudio 500/1
Speaker Amp-Pioneer GM-X564
subs-pioneer Primer TS-W1241DVC
pioneer primer TS-C504 component speakers