If it weren't for the whole KNY craze going on, people would be talking a lot more about SxF's impressive numbers.
Let's talk about SxF a little then.
Unless I'm forgetting something, the fastest Volume 1 to have reached 1m units sold (not shipped, so since Oricon started giving sell-through data) is
Haven't You Heard? I'm Sakamoto's, which reached 1m in 48 weeks.
(Can anyone confirm that it is indeed the holder of that record? I also wonder which manga is #2 if it is, probably Assassination Classroom?)
Using Shoseki estimates, SxF's first volume should be around 809k as of April 26th. It would need to do ~39k per week for the next five weeks to beat Sakamoto (SxF's 48th week ends on May 31st), so it seems highly unlikely that it can beat it.
However, reaching 1m in a year (52 weeks) seems possible.
There are 9 weeks left until that point, meaning it would need to sell an average of ~21k per weeks to reach 1m in under a year. With the upcoming 4th volume and the reprints that just arrived, I think it can do it.
Its weekly average for the 17 weeks of 2020 is 18.3k (however it sold quite poorly these past two weeks
(stock issues maybe?), since its average for the first 15 weeks of 2020 was 19.7k per week)
Finally, If it manages to consitently sell a million units per volume, then starting 2021 it will be the second best-selling manga currently running
(Attack on Titan and Demon Slayer will be over, so it will just be behind One Piece. I'm ignoring Hiatus x Hiatus, but even that isn't guaranteed 1m units sold anymore). Unless Kingdom rises to a higher level, I guess
And that's without an anime