This is impossible
Switch japanese userbase is only 13 million players atm. They will need to push this much further, something like 29-30 million users, DS-like userbase
Even if Swtich eventually archive 25-26 million users, it will take years. By the time it happens Animal Crossing will be a really old game (3, maybe 4 years old). New Leaf sold around 66% of it's lifetime sales on the first year, so if New Horizons make the same cut it would lead for a 7,92 (66% of 12 million) million copies in a year, it means literally more than half of switch owners
There is a roof for NH sales, the roof is the number of Switch players. I don't think it can move anything higher than 6-6.5 million copies this year. And god, 6+ million copies is already freaking insane and would lead form something around ~9 million copies lifetime, literally the best selling game ever in Japan.
It looks like not only will it hit 8 million but could hit 9 million in the first year. I would've thought it would be impossible too but lesson learned, don't doubt tbone with regards to Animal Crossing lol.