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.
The length of the JoJo manga varies depending on which part you consider. Some parts are shorter, while others are more extensive. In total, it's a substantial body of work that has captured fans' attention for years.
It's hard to say for sure, but Thanos with the Infinity Stones could be extremely powerful. However, if we consider the JoJo universe, DIO with his time - stop ability and vampiric powers is also a force to be reckoned with. If they were to face off, it would be a battle of epic proportions.
It could be Jon Snow. If he has a Stand in the fanfiction, combined with his already proven leadership skills and combat abilities, he would be a force to be reckoned with. His ability to come back from the dead might also be related to his Stand powers in some way, making him even more formidable.
The concept has a significant impact on the JoJo fandom. Firstly, it deepens the connection between fans and the characters. By imagining characters reading fanfiction, fans feel closer to them. Also, it can inspire new trends in fanfiction writing. Writers might start creating stories specifically with the idea of a character's reaction in mind, leading to more diverse and interesting fan works. Moreover, it can increase the sense of community as fans share their ideas about characters and fanfiction.