How many main story "parts" does JoJo's Bizarre Adventure have?

How to get close to this number

Divide the run by the part length. The manga began in 1987 and has been serialised continuously since, and each part runs several years before the protagonist changes. Three and a half decades divided by roughly four years per part gives a count of around nine, which is exactly right.

The structural device is what makes this countable at all. Rather than following one character indefinitely, the work restarts with a new protagonist from the same bloodline every few years, which turns a single long serialisation into a sequence of discrete stories. Very few long-running works do this, and it is why "parts" is a meaningful unit here and not in most manga.

The count is still moving: the most recent part began serialisation recently and is ongoing, so this is a snapshot of a work in progress rather than a finished total. Any count for an active serialisation needs its date, and this one will be wrong eventually by design.