Seaton is all about the smile factor.
It all depends on your budget and how many subs your room's decor can suck up.
Two subs better than one, three are better than two and four are better than three. It's all about spreading the sound waves around so as to overlap each other and not cancel each frequency sound wave out. Makes for smoother bass room sound.
The common choice is to go with two subs. In the end, for most, it's about the smile factor.