g.costanza: Your room size of approx 1,800 cu. ft. (if you have 8-foot ceiling height) places it just outside the small room size of up to 1,500 cu. ft. I'd suggest getting the largest size driver your size restrictions allow (I'm not sure how the Dayton SUB-1200 with a 12-inch driver fits into that scenario). Normally, I would advise to start with one sub and find the best sonic location, then go from there. Since you want to start with two (or four), I would start out with two first. More subwoofers won't give you deeper bass, but will afford you a little extra headroom (3 - 6 dB for each sub, depending upon location). It also helps smooth out uneven frequencies at the listening position.
The optimum location of one subwoofer is the easiest to find (via the sub crawl). With multiples, it isn't as easy and takes time and effort (and often a mic and REW software to graph out the room -- or something similar). Multiples can create their own set of problems, like cancellation, but with patience you can be rewarded.