Just like American comic book issues are eventually collected and sold in a trade paperback, individual manga chapters that are initially released inside manga magazines are eventually collected and sold in collected "volumes." Here are the top 20 selling manga volumes in Japan for the month of October (technically (October 2 - November 5),
courtesy of MAL.
*1. 1,505,639 One Piece Vol.87
*2. *,688,666 Kingdom Vol.48
*3. *,640,913 Haikyuu!! Vol.28
*4. *,601,652 Tokyo Ghoul:re Vol.13
*5. *,420,350 Nanatsu no Taizai Vol.28 (The Seven Deadly Sins)
*6. *,392,253 Ao no Exorcist Vol.20 (Blue Exorcist)
*7. *,311,651 3-gatsu no Lion Vol.13 (March Comes in Like a Lion)
*8. *,274,338 Boku no Hero Academia Vol.16 (My Hero Academia)
*9. *,273,161 Magi Vol.36
10. *,220,408 Gintama Vol.70
11. *,204,819 Skip Beat! Vol.41
12. *,194,163 Black Clover Vol.13
13. *,187,484 Shuumatsu no Harem Vol.4 (World's End Harem)
14. *,183,907 Grand Blue Vol.9
15. *,180,410 Giant Killing Vol.45
16. *,179,371 Major 2nd Vol.11
17. *,174,721 Shokugeki no Souma Vol.27 (Food Wars!)
18. *,153,742 Haru Matsu Bokura Vol.8 (Waiting for Spring)
19. *,150,889 Dragon Ball Super Vol.4
20. *,144,360 Yakusoku no Neverland Vol.6 (The Promised Neverland)
Certainly seeing One Piece at the top of the list should come as no surprise as a staggering 1.5 million copies of One Piece Volume 87 were sold during the month. The volume was just released in Japan on November 2, barely making the cut but as you can see, it didn't need any extra days for circulation. Also of interest, is the fact that the top 10 have all had anime adaptation at one point or another. It isn't until the #13 spot where we find a manga that doesn't have an anime adaptation yet, and that's likely because World's End Harem just began serialization in Shonen Jump Plus on May 8, 2016.