Why do JoJo volume numbers start over?
Collect the Japanese volumes and the shelf turns on you at Part 6. Parts 1 to 5 count straight up, volume 1 to volume 63. Then Stone Ocean restarts at volume 1, and every part after it counts itself: Steel Ball Run 1 to 24, JoJolion 1 to 27, The JoJoLands from 1 again. The overall count never stopped. It just moved into smaller print.
THE COVER THAT SHOWS BOTH NUMBERS
Stone Ocean volume 1 is also volume 64
THE TWO COUNTS, SIDE BY SIDE
The collector's mapAfter volume 63, the last Golden Wind volume, the tankobon numbering for each subsequent part restarts from one. The overall sequence underneath runs unbroken: Stone Ocean's 17 volumes are 64 to 80 overall, Steel Ball Run's 24 are 81 to 104, JoJolion's 27 are 105 to 131, and The JoJoLands begins at 132. A listing that says only volume 1 can mean the first book of Parts 6, 7, 8, or 9, four different books at four very different scarcities, which is why the part name, not the number, is the identifier that matters.
Other Japanese editions count differently again: the compact bunkoban reissues of 2002 to 2009 renumbered Parts 1 to 6 in their own sequence, with Steel Ball Run following in bunkoban in 2017 and 2018. Any JoJo number is meaningless until the edition is named with it.