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

Blast that room dictator! Who does he think he is to tell you what you can and cannot do?

Yeah, he’s a bad dude just like his evil brother the cabinet dictator.  Really hate that guy too.  Lest we not forget the mother of them all — the wife dictator.  Dedicated rooms and divorce are the best weapons we audiophiles have against such injustice.  Yeah they’re expensive, but you get what you pay for. 

I have an SVS SB2000 (sealed box) that has a 12” driver and goes down to 19Hz within 3dB.  I have a similarly-sized room that opens to two medium-sized rooms, and with the right movie the floors literally shake so I doubt you’d have a problem with the “enough” part.  You don’t really give enough detail to go any further than that though.  FWIW.