I'm no expert and I don't disagree with what others have said but my vote goes to 2 18s or 3 15s.
Largely dependent on what you listen to regularly but with a system this size I don't imagine you're going for tight drum kicks and snappy little punches. I was always told to stay away from 18s cuz they would sound "sloppy" or "slow" until I got one at some point and was extremely pleased because the only time I really want the subs to play at full volume are long rumble notes. Half volume on rock etc were plenty accurate.
I'm also curious what your amp/electrical plans are.
I imagine 2 18s on 4000 watts (2000 each) would have a lot more excursion than 6 12s (666 watts each)
Even if the 12s have more cone area, the 18s would displace more in the end due to the extra excursion.
So correct me if I'm wrong somebody but I believe 12s might be better being as accurate as possible but larger cones will get louder.
Guess it depends what your goal is.
2 death rows would be dirty in my opinion
Again. Not an expert. Other people in this forum have more years of experience.
Did everyone hear that. Not an expert