Since they announced a sole writer (Gillen) and one penciler (Schiti) for the event, it is probably a minisseries. I would also say something around 6 issues based on recent events.
X-men, Immortal X-men, Eternals and Avengers will get tie-ins in their own monthly books. And there will be probably some other one-shots and minisseries as tie-ins, as usual.
Aaron's Avengers is unreadable, I'm certainly ignoring that. I blame myself for everytime I gave that book a chance.