My heart says 5 million, my brain says 2.5... I think that averages to... 3.75? I'll go with that.

I think it'll sell over 2 million.

Seems like a tough proposition to me, but hopefully you're right. I don't think a single Metroid game has ever come close to those numbers, has it? I feel like the sales data for Metroid titles is very elusive. 

VGChartz has the highest selling game in the series as Metroid Prime 1 at 2.82 million.

i'm going with this numbers as well, of course it could also fuck up and going bellow 1.5 - 1 million that was the average for other not most successful games in the series.

OP, how about adding a poll, to help us track where most feel it will fall.
Will be interesting to look back on in a year's time (or more) to see how the sales compare to the poll results.

Added

I’m predicting less than 2 million, but will be very happy to be proven wrong.
If it ends up out selling Hollow Knight I’ll be happily astonished.

I’m also thinking 2M to 2.5M. It’ll get the bonus for being on Switch but at the same time a penalty for being “just” a 2D sidescroller. So that means it’ll end above the average for mainline, numbered Metroids, but won’t be the top selling.

EDIT: I forgot to mention, I don’t think the pricepoint, whether 40, 50 or 60 is going to matter very much either way. Fans will buy no matter what and the series itself is too much ‘core’ to be very attractive to ‘casuals’ regardless.

2-3 million units seems good enough. While Metroid games aren't big sellers by any means, the Switch has managed to break many sales records in regards to its 1st parties, so I can see Dread doing very well, despite it's higher price. Plus, it's a new 2D Metroid after so long.