I'd guess that it will have difficulty breaking 1 mil. It might get there, but it wont be quick and it wont be by much. At the end of the day, Arms is still a fighting game which tends to be a smaller market than most genres and from what we've seen, it looks fun but it doesn't seem to have a ton more too it.
That said, I've been wrong before (Splatoon) and I'd love to be wrong again.