If your main speakers reach down to at least 30Hx with low distortion, you can get away with one great sub by running your mains full-range and using the sub just to augment and extend the mains. This gives you three bass sources for most music. Otherwise, you're a lot better off with two subs. For the case where you run the mains full-range you'll also need some sort of equalization to tune the sub's response so that it is just not additive to the mains, otherwise you'll just have bloated, muddy bass octaves.
What are your main speakers?