Hey guys,

How would you go about deciding what size, and number of subs you need?

Im running an '86 Buick Regal, so I have a ton of room. Was planning on pointing the subs vertical towards my rear speaker holes (have been removed).

I was going to get two 12" subs, but Im starting to think that parhaps I should just go with the 10"?

I would like some kick, but nothing that makes me seem like Im trying to compete with others, if you know what I mean.

 

I'd say do the 2-10's.
Combined, they'll have more "cone surface area" than a single 12, and use approximately the same cabinet area.
The 12 will go down lower in frequency, but not by that much 10Hz. typically.
The 2-10's will allow for different wiring configuration and therefore more flexibility.
If you can,try to design the cabinet, so that the output of the woofers(and port) is "vented into" or ducted to the rear parcel shelf(deck), and through the old 4x10 holes, and try to increase the size of those holes, without compromising the structural integrity of the car(in case of roll over).
This will prevent a rattling trunk, and allow all the bass to stay in the passanger compartment.
Open the window, and you are in a bandpass.