How many guitar tracks?

Taste great!
NO less filling!

Just like bud light.
Like somebody else said try quad and dual and see what is best for the material. I don't quad track because I am lazy as fuck and play sloppy as a drunk sorority chicks cottage cheese filled axe wound. Plus I love Exodus atrocity exhibit guitar tone and that is just two tracks.
 
I like quad tracking since it forces you make sure your chops are up to scratch.
In general, something like thrash metal, I think is best with 2 tracks, but something with more groove, like Nevermore I guess, I love how that stuff sounds with 4 tracks.
 
i usually start with double tracking, and if it's missing something you will know. some music sounds just right with 2 gtrs, other music really needs the quadtrack thickness to get the right sound. it tends to be pretty obvious whether it's working for the style or not.

edit: btw, i don't quite agree that quad tracking requires better chops......it's true that you need to play tight so it doesn't wash out, but on the other hand there can be some slight inaccuracies in each guitar track that get's masked by the other tracks - which is not the case with double tracking. if you're double tracking and you hit a bum note, people will notice moreso than with quad tracking.