I'm going to guess 5-6.9M. Most of the 3DS releases almost hit the 5M mark, and this will be available on two different platforms. If there is no "ultimate" version, and instead a large expansion for the game I will bump up my prediction to 7-9.9M. Most of the 3DS releases almost hit the 10M mark if you include their ultimate variants, though that also includes those who made duplicate purchases; hence why it isn't doubled.
I'd love to be proven wrong though. There are a lot more Monster Hunter fans now than their were on the 3DS.