2020 Annual Manga Sales Ranking Announced by Oricon.

Related rankings are compiled here
Oricon Comic Ranking Summary

The top spot was taken by the completed series "Demon Slayer."
The TV anime that aired in 2019 received a tremendous response,
and in May 2020, the main story, which spanned 205 chapters, concluded to the dismay of fans.
Subsequently, in October, the release of the movie "Demon Slayer: Mugen Train" quickly broke records, surpassing 10 billion yen in box office revenue within just 10 days of its release.
Ultimately, it surpassed Studio Ghibli's "Spirited Away," recording 40.4 billion yen in Japan and becoming the highest-grossing film of all time in the country.

In 2020, "Demon Slayer" achieved an unprecedented feat by selling 80 million copies of its comics, an extraordinary number about ten times that of the second place "Kingdom."
In 2020, "Demon Slayer" indeed became a social phenomenon.

2020 Annual Manga Sales Ranking

Ranking / Estimated number of copies sold / Title / Author

1 82,345,447 Demon Slayer Koyoharu Gotouge
2 8,251,058　Kingdom Yasuhisa Hara
3 7,709,667　ONE PIECE Eiichiro Oda
4 7,212,099　Haikyuu!! Haruichi Furudate
5 6,702,736　Jujutsu Kaisen Gege Akutami
6 6,368,445　The Promised Neverland Kaiu Shirai, Posuka Demizu
7 6,145,591　The Quintessential Quintuplets Negi Haruba
8 6,003,589　My Hero Academia Kohei Horikoshi
9 4,541,589　SPY×FAMILY Tatsuya Endo
10 4,306,012　Attack on Titan Hajime Isayama