I reckon it'll do between 1 and 2 million, probably around 1.6m if I had to guess to the nearest 100k.
Nintendo are giving it a decent push, and Xenoblade 2 shows there's a market for JRPGs on Switch. On the other hand, I think due to its intentionally retro style it won't quite be mainstream enough to do more than 2 million.
Would be interested to hear from those who voted "less than 1 million" as to their reasoning.