I think this will be very subjective on the answers you get. I think it all depends on what kind of sound you are looking for.
1. What kind of systems have you had in the past? Have they all been heavy bass or not? If they have been I would say the more subs the better.
2. Use a car audio shop that you have a relationship with and they know what you like. If they suggest something you should trust them.
I personally am putting one JL Audio 10" sub under the back seat. I am not looking for huge bass, I just want a little more than the factory sent me
