4-5 million, might be 6 if do a switch+pc release. the story will confuse many newcomers which is gonna hold people back from getting it.
public demo will help for people to at least try the gameplay, it can add more sells, if they do one..
I think a PC release is doable but in terms of how much power it appears to be using so far I think a switch port is impossible without REALLY scaling down the game... sure it can run HD but it's nowhere as powerful as the XB1 or PS4 because the switch's gimmick is that it doubles as a portable system and with the performance drops from going portable I don't even think you'd be able to play KH3 on the go. it would be the first main line kingdom hearts game to be played on a nintendo console yes but to try to port over KH3 to the switch they'd have to really scale it down which makes me think they'd have to really up the specs on the switch to run KH3 on the go because of the performance drops. sure nintendo makes great games still but I think KH3 would be too much of a big pill to swallow for nintendo