How many subs do I need?

Well, typically, you'd want all your subs to be the same for balance. Going bigger to the 12" subs would only help the situation I'm assuming since you have not given the model of the sub. Also what is your room size you're trying to fill.

I can only tell you of my experience. I had one ULS-15 (15" sub) and then added a nearfield Infinity 12" sub and I can tell you that it added a world of difference as far as evening out the bass in the room. The only thing is, the 12" sub was a cheaper model and so it was not as clean or accurate as the ULS-15. Bass output and mid-bass more importantly sounded better, but I'd say it was boomier and if you set the sub gain too high, it drowns out the music. When I got my 2nd ULS-15 this week, I noticed a much nicer blend of bass that is much more accurate and tight. Gone was the boominess. Bass is still evenly distributed from what little I have heard it so far. So, that is my experience with mixing and matching subs.

Two like subs, at least. Properly setup, that will get you 90% of the way there.

8" subs are NOT subs.

Awesome, Jon. Do you still have the Infinity 12" sub? if so, try this.

You can try the Geddes method I posted above. Put the two 8's together (acoustic coupling) for best effect, maybe try them behind the couch. You won't need much volume out of them to do this.

This method is for domestic rooms, not outdoors.