I’ll second what Rob said, and I would also like to respectfully suggest that one really good $1000 subwoofer will usually sound better than two subwoofers that cost $500 or four subs for $250. Of course, this is a very general statement, and it depends on a ton of other factors, but I believe that it holds for the majority of cases.
I would also like to bring up the wife acceptance factor as most of us married guys have a hard time getting one subwoofer into a room, never mind 2 or 4. If this is a factor, I would suggest one bigger subwoofer is easier to get approval for than 2 or 4 smaller ones. Custom millwork helps a lot, as if the sub(s) are out of sight it certainly helps with the décor factor, but does limit which subs you can put into said enclosure, and obviously where they go in a room. I had our carpenter build a shelf unit that had space for two subwoofers. I also made the enclosures big enough that I could put in some fairly substantial subwoofers down the road if budget would allow.
Interestingly enough, I have never ended up keeping a second subwoofer in the second spot. I started with one 10 inch sub, and did eventually buy a second unit. The second sub did not give the impact I was hoping for, so I returned it for a 15 inch sub that nearly doubled my budget (thank you Visa). While I tried both subs for a while, it was total overkill, so I sold the original 10 inch sub, and kept my system with the single 15 inch sub.
I have often wondered what sort of structural damage I could do with two 15 inch subwoofers in that room, but so far I have resisted temptation.