Considering how much bigger Uncharted 4 ended up selling compared even to Uncharted 3, I think it's more than possible that this game will get 8-12 million sales.
I mean, I'm reasonably certain 2-3 million is where it'll sell first week, and that 5 million is pretty crazy, but I'd be happy to see it if that happened. It's a great game and I think we need more like it. Good sales means more incentive for other companies to follow in their footsteps.