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

That's a good sized room.  Even if you went with a larger sub I would probably add a second one to the mix.
Depends on which sub and what type of bass you are looking for too.  Music, Home Theater, combo of both.....etc.

I'm using an SVS SB2000 (sealed box) sub in a 320 square foot room with 8' ceilings, in a 2.1 stereo application. This sub is more than adequate for this application. Room placement will be important.

I ended up buying the SVS wireless adapter for the sub, and playing around with room placement for a few weeks until I found the spot where it sounded best to me, and blended well with my mains.

If I could do it all over though, I would go with 2 smaller / identical subs like many people are recommending.

One sub, ok Tekton 21" bass bin (sub) it has two M/B couplers in it to...

Great box  25 to 300 or so,, 95% + sensitive. 

Regards..