The number of JoJo mangas can be a bit hard to pin down precisely. There are the main series volumes, along with special editions and related works. But you can expect a substantial collection over the years.
It varies. Some spinoffs might build on the main canon or offer alternate perspectives, but not all are strictly considered part of the core JoJo universe. It often comes down to how they tie into the main plot and characters.
Echii mangas usually have more adult-oriented content like sexual themes and imagery compared to regular mangas which may focus on a wider range of genres and themes.
There's no way to really tell if it's common specifically for someone named Jojo. However, in general, reading fanfiction is a popular activity among fans of many different franchises. The Jojo series has a large and dedicated fan base, so it's likely that there are fans named Jojo who partake in reading fanfiction, but we can't say for sure how common it is without more data.
Yes, mangas can be considered graphic novels. They share many characteristics such as being a combination of text and illustrations to tell a story. Manga typically has its own unique art styles and narrative structures, but like graphic novels, it uses sequential art to convey a plot, develop characters, and create a fictional world.