Repost from other thread for exposure.
I think it sold around 3.8m.
~350m= 5.2m Halo 3 for just software.
~400m has some subtractions from hardware and microtransactions.
A year ago Halo sold 60m. The franchise got 5bn with Halo 5. Without Halo 5 it is 4.6bn.
Using proportions we get 5bn x 60m / 4.6bn = 65m
That suggests 5m but it is obviously less due to Halo sales within a year and the PR suggesting it is both hardware and software this time.
So my rough guess is 3.8m-4m