For sure the last device to hit 7 million was DS in 2007...
As a reminder, although there were many games that sold 1M on the DS that year, nothing can compare with what Animal Crossing New Horizon will end up selling by the end of the year.
Top 10 DS 2007
1 Pokémon Mystery Dungeon: Explorers of Time / Explorers of Darkness - 1.256.516
I'm not sure how much Software Switch will sale this year but we are already at 8M for the Top 10 with another 6 months to go... very early prediction without even knowing what to expect until the end of the year.
Animal Crossing >7M
Ring Fit Adventure >2M
Pokemon Sword/Shield, Clubhouse Games, Breath of the Wild 2 >1M
So from the Top 5 I'm already looking at 12M, rest of Top 10 at 750K on average would mean a Top 10 of 15.75M. Depending on when Pokemon Snap launches and how it's received it's another title that coult hit 1M this year.
Even if Animal Crossing and Ring Fit end up doing less due to lack of supply, it will still be massively bigger than the last huge year DS had.
Nothing will ever come close to topping 2006 however...
I mean, 2007 is just spillover from 2006's lineup.
2006... yeah combine mainline pokemon for a new gen with a 2d mario title AND an animal crossing title.... on a handhled... in japan.... that just guarentees 15+ mil.
If they want 7 mil, I can see one thing that can potentially confirm a 7 mil sell though, perhaps even 8 mil.... a proper new traditional mainlaine MH title.
come on capcom that just money sitting on the table, that would sell at least twice as well as MHW in japan.