My room is similar in dimensions to yours. "Enough"? That depends on how much thundering bass you want. A room of that size must be able to accommodate at least a second sub. Don't predetermine where a second sub must go. Sub placement is one of the the most critical parts of the deal. Surely, your room size and geometrical dimensions can allow for the addition of at least a second box.
Example....In my room, equal in size to yours, I use a pair of bookshelf speakers on stands and a pair of 10" subs. Fills my bill perfectly. My listening is primarily music rather than HT. If you're unsure whether one sub is enough start with one. See how it works. A second sub can be added later. But, don't obsess about the size of the sub speaker. Focus on the placement of it/them. If music, movies, or both are your preference that will guide you in one direction or another.
I have benefited from millercarbon's suggestions about subs and other things. I would ask him directly what results you want to obtain.
Example....In my room, equal in size to yours, I use a pair of bookshelf speakers on stands and a pair of 10" subs. Fills my bill perfectly. My listening is primarily music rather than HT. If you're unsure whether one sub is enough start with one. See how it works. A second sub can be added later. But, don't obsess about the size of the sub speaker. Focus on the placement of it/them. If music, movies, or both are your preference that will guide you in one direction or another.
I have benefited from millercarbon's suggestions about subs and other things. I would ask him directly what results you want to obtain.