I'd love if they release the whole thing in one trade, but knowing Marvel, it'll be three overpriced volumes.
Possibly, though I think they may do two. Marvel's solicitation for Avengers Vs. X-Men
#5 calls it "fiery conclusion of Act One," so it would make sense to do issues 0-5 (six issues) in one trade, then issues 6-12 in the next.
Marvel will definitely do Premiere Hardcovers first, though, and then trades four months later, and then possibly an omnibus hardcover eighteen months after that, because that's the way Marvel rolls.