Death Note: is probably gonna get there eventually (It had 30million with only
12 volumes 13 volumes and that was information from 2015, I can only assume maybe it's grown a bit over 5 years). Plus is a classic. Plus is a "short" read. Plus it has an interesting concept. I definitely think it will be one of those stories that will continue to sell for a long, long time.
Dr. Slump: had 35'000,000 with 18 volumes (So around 2mill per volume), this info is from 2008, so again 12 years have passed it likely has gained a couple million sales, but I guess it still hasn't reached 3mill per volume.. Maybe it never will? Who knows.
contains pocket edition and deluxe.
Yu Yu Haksuho: has sold 50mill with 19 volumes (Average 2.6 mill) and this data is from 2014 so it's probably getting to 3mill soonish (IIRC it was released in Bunkoban format after 2014... I think, not sure, but if so it probably boosted the series sales by a bit more.
Honorable mentions:
Naruto: 140mill with 72 volumes (So around 2mil average, Info from 2015), it's probably gonna reach 3mill per volume in its lifetime I guess.
From may 2019 : 153 000 000 for 73 volumes (72 + gaiden)
Dragon Quest Dai No Daiboken: So this is a very special case because this manga has 50million in sales with 37 volumes
BUT the last time we got info on this one is 2005 (Again all this is info I got from easylove's video) so the info is not updated, I think if this info is correct it has probably sold an extra 10million?? Is that fair to say?? This is just my speculation of course.
Anyways Dai No Daiboken is also getting a (Hopefully full) remake this year which is gonna lead to a re-edition (Hopefully a kanzenban, pretty please Shueisha! I love color pages from the 80s/90s)
AND it's a very popular series outside Japan too, so obviously it's gonna get licensed worldwide so it's gonna increase it's total sales (In fact this has been probably the most requested manga for the Spain community for the last.. decade maybe? I think Spaniards here can confirm it, the publisher over there has basically been blue balling them for like 5 years licensing every damn Dragon Quest manga BUT Dai no Daiboken haha).
What I'm trying to say, DQ DnD currently should be at around 60ish maybe 70ish mill.. And the remake is probably gonna make it sell more and I know these are lots of stretches and it's pretty unlikely DQ DnD will ever reach the 3mill per volume but I think depending on all these factors it seems doable.
And now the series that have sold 3mill or more average per volume:
Hokuto No Ken 100 million with only 27 volumes (Info from 2015 btw so it has sold more probably)
(its 60 000 000 for Japan (including all editions). 100 000 000 its world numbers. )
Slam Dunk 121 million with 32 volumes (info from 2014 so I assume it must be at around 130mill right now or even more since it had a reprint in a new format a couple of years ago), more info below.*
Dragon Ball I'm not completely sure of DB's sales... Outside or Inside of Japan, or global sales.. EasyLove's video has it at 160million,
this article of compress from 2008 has it at 150million, IIRC DB's worldwide sales are 300million.
Anyways if we go with Japan-only numbers: 160mill (Probably more now), 42 volumes, average of 4million-ish per volume
If we go global sales it's reportedly 300million, with 42 volumes that would be 7.1 million average per volume.
World numbers is 250 000 000 (november 2018). The 160,000,000 figure logically includes the deluxe edition figures (20,000,000). So we would be 140,000,000 for the 42 volumes.
One Piece Easylove's video had it at 362million (Thru volume 87), so diving 362mill/87= 4.1million-ish per volume.
One Piece : its 390 000 000 for 95 volumes (in Japan).
Global sales are 470 million copies with 96 volumes so 4.8million per volume.
Edit: Underlined info was provided by
@Heiji, thanks for your corrections

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Now something very important to consider is these are all series that have reached these numbers after decades of being published (So selling backlog for years and years)
AND Kimetsu is not gonna stay at 3 million as much as I hate it lol.
If I were to venture a guess.. Maybe only One Piece and Dragon Ball reached a 3 milion average per volume during their first 5 years.. Maybe Slam Dunk too? I mean there must be a reason why Slam Dunk had the record for most printed 1st printing of a volume before One Piece broke it.. Idk.
For example Slam Dunk had sold 100 million by 2004 (As in.. It took Slam Dunk 14 years to get to 100million sales, which is pretty much 3 million average per volume) and then in 2012 it was reported it had sold 118 million.. And 121 minion by 2014..
And Kimetsu reached the equivalent of Slam Dunk's 100 million (3 million average) in the span of .. what? 5 years?
Maybe Kimetsu is one of those things that has reached most of its audience, but I still think its short length will definitely make it a "shonen classic" and a "must read for shonen fans" (Akin to Yu Yu Hakusho in a lot of ways, I think). so it will continue to sell in the long run too.