It's definitely hard to predict. I voted 15m-17.4m, but I can see it going even higher. Check out these legs:
December 2019 - 2.17m/2.17m
March 2020 - 0.56m/2.73m
June 2020 - 1.17m/3.90m
September 2020 - 1.94m/5.84m
Last quarter sales were almost as good as launch quarter. This quarter sales are likely to be higher than launch quarter. The easiest mistake to make in projecting RFA's sales is to think it will sell like most other games. This is a Blue Ocean title. It's selling by word of mouth to people who would normally not get a Nintendo console. COVID gave it a boost, but that doesn't mean it's a temporary boost. Instead it's more like free marketing to people who normally don't buy consoles. Now that word of mouth has gotten out about it, it's going to keep selling even after COVID is over (and COVID will still be around in 2021). I may very well be short changing it at 15-17.4m.
Bonus: Don't have this yet, but I am likely to get it at some point.