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

Enough for what? It'll add bass for sure. But you'll end up overdriving it to get as much out of it to fill the room creating a hotspot leading to imbalanced sound. Best to use two, even if they're smaller. 

Using one subwoofer has always worked out fine for me!

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.