Will 12 inch subwoofer be enough for an 18x20 room (with 10 ft cielings)

I've used 2 x 18" Eminence Kilomax drivers is 14 cu ft sealed cabinets in a room of similar volume.  EQ'ed the subs and placed them in the corners behind the main speakers.  Works OK. 

It all depends on the listening position in the room.  The optimum listening position and main speaker position may not be the optimum listening position for the subs.