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

FWIW, I have a large room with one sub. REL S3. It's fine but I find what I wrote above to hold true. I'm saving for another.

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. 

Room layout dictates I can't have more than one sub.

Blast that room dictator! Who does he think he is to tell you what you can and cannot do? Oh well. We get the government we deserve, I guess is true after all.