6 12s
BUT the box will have to be about 12cubes and about 120sq in of port tuned at 30hz for nice flat response with a good deal of output.
Ive messed with SA-12s enough to know what they like.
I have my 6 SA12's in 12 cubes with 130" of port tunes to 34 and it hammers. I have played with port sizes from 87" up to 150 and the I agree with Eric about the 120-130 range. Tuning is a different story, I dont see a need to tune that low, but to each there own..
I assumer you're in a suv do you goto the windowline or to the headrest? Does it hit hard 55hz to 65hz?