The adage is "no replacement for displacement", which is cone area X excursion (also includes the volume of air vibrated in your port).
You may or may not benefit from more motors. On the meter you'll find more Jeeps get impressive numbers with 2 18s than multiple smaller subs. I'm not sure the science on this, but cone area isn't always created equal especially if you're planning to wall.
Yes, you get more power handling, but Jeff makes an excellent point about adding much more weight and more complexity.
Before I walled I had used single 15, pair of 15's pair of 12's, three and four 12s and 4 10s and produced roughly the same numbers on the meter with all variations. Frequency response was quite different though as the subs behave a bit differently with different cones on them (assuming same motor). I preferred 3 12s but that might just have been me getting lucky with the box or dependent on my particular subs either.
In short, do what you like, for what you're doing the real tradeoff will be weight and complexity for power handling, just playing music loud and impressing your buddies will all wash about the same. Also be sure you can fit all of them in your baffle along with adequate port. 12s or 10s will likely only allow center horizontal slot port. If you want to keep your box simple you'll only have about 33" up and down and 40-42 side to side. Even the 15s is a tight squeeze.
Get yourself a big *** box (rent a center or similar place usually has big TV boxes they're throwing away) and mock up a front out of cardboard. Allow a few inches on every side for side walls, and see how much real estate you really have to play with.... also figure whatever you think you'll have you probably will still have a bit less when it all comes together.
