Currently One Piece has sold 350,000,000 around the world. 300,000,000 alone in Japan, so rounding things up 4'200,000 give or take per volume.
Now I think that right now the best selling manga is Astroboy which according to wiki has sold around 100'000,000 copies, only having 23 volumes that rounds it up to 4'300,000 per volume. Also note that wikipedia's source claims this 100'000,000 way back from 1990 so is expected to be a little higher now.
Astroboy has also been in print since the 50's, so it achieving those sales over the course of sixty years is nothing to scoff at, its not nearly as impressive. Its sales are far, far behind OP's at this point, it's nowhere even close. (Dragonball is the second place runner up, incidentally.)
As for final sales on OP? Depends on if the series runs for another 10 or 15 years. Currently its FIRST PRINTING is 4 million copies a volume, 4-5 volumes a year, plus back issues, it adds about 30 million copies annually now.
By the time its done first prints will probably be up to 5 or 6 million. But even just at the current rate, for another 10 years, would add another 300,000,000 in Japan.
So absolute bare minimum it'll hit 600,000,000 before it's done. But if it runs for 15+ more years, the starting print runs keep rising, or theres another jump in interest like there was during the war, or it picks up more in other countries? It'll probably far exceed that by the time the series ends… (A 15 year run plus rising excitement when it nears the end? Hitting 800 mill+ isn't unreasonable by that point.)
...Let alone the decades it remains on shelves afterwards.
(And thats not even counting databooks, colorwalks, and other merchandise.)