curl-6 on 31 March 2021
If World launched with 5 million, then it's currently just over double that on PS4 + Xbone after three years according to the leaked numbers, so similar legs would see the Switch version a little over 8 million.
That would still put it above all but a handful of third party titles in Nintendo console history.
Still, I've learned to never underestimate the Switch so you never know.
