one 12in sub or 2 8 inch subs

I'd personally go with a pair of svs sb12 nsd's. I had an svs sb12 plus which was the predesessor to the sb12 nsd and it was a great sub. I would have kept it but the color didn't match the rest of my gear, and I was having a hard time finding another one (I wanted two because two gives a more uniform sound with more headroom). So anyways, I sold it and when I'm in the market for subs again, I'll probably get a pair of the svs sb12 plus'. I remember seeing awhile back that they had a slight discount if you bought a pair; might be worthwhile to ask them. All in all, my svs played pretty low (22 hz?) and was pretty fast and the new sb12 nsd is supposed to be even faster with less overhang. I don't think you'd be disappointed in any way with a pair.

Ya know this doesn't have to be as confusing as people tend to make it. Do as Duke suggested and understand what he's saying. Not that you don't know but just in case. And your chances of success will greatly improve.

I have no afiliation with Duke and have never met him, but he's got this subject covered as well as I could have said it.
Good luck

An old rule of thumb;

(2) 8in subs are equivalent to (1) 12in sub.

(2) 10in subs are equivalent to (1) 15 in sub.

With either double 8 or 10 inch subs, the bass is much more refined vs. a single, larger driver IMO.

Additionally, the larger , single driver requires more air pressure to be moved, for better sound.

Happy Listening!