I don't have the exact count. Sometimes the number of comics in a volume can vary depending on how they're packaged and distributed. You might have better luck reaching out to comic book publishers or libraries that specialize in comics.
Obviously, Spider - Man himself, Peter Parker, is one of the most popular characters. He's the central figure, and all the stories revolve around him. Another popular character is probably Mary Jane. She has a special relationship with Peter, and her character often adds an emotional depth to the stories. And of course, villains like Venom are also very popular. Venom's menacing presence and his complex relationship with Spider - Man make for great fanfiction material.
Perhaps a plot in Ultimate Spider - Man Vol 3 Fanfiction is about Spider - Man getting trapped in an alternate dimension. In this dimension, the rules are different, and he has to figure out how to get back to his own world while also dealing with the strange inhabitants and new threats there. It could also be interesting if there was a plot where Spider - Man has to deal with a virus that affects his powers, causing them to be unpredictable. He then has to find a cure while still protecting the city.
The Green Goblin is a classic choice. His maniacal nature and his flying glider make for some exciting action scenes. Mary Jane could also be included. Her relationship with Spider - Man is an important part of the Spider - Man lore. And don't forget about Aunt May. She can add some heartwarming and sometimes comical family - related moments to the fanfic.
Maybe a plot where Spider - Man has to face a new villain who has the power to manipulate time. This would make for some really interesting battles and strategic thinking on Spider - Man's part. He would have to find a way to defeat the villain without getting caught in the time - warping chaos. Also, a story about Spider - Man traveling to an alternate dimension where his powers work differently could be cool.
One interesting aspect could be the unique interpretations of Spider - Man's character. Fanfics often explore different sides of well - known characters. In this case, it might show a more personal or emotional side of Spider - Man that we don't see in the official comics or movies.
One common theme is probably power and responsibility. Just like in the original Spiderman stories, Peter has to deal with the consequences of having great power. Another theme could be identity. He might struggle with his dual identity as Peter Parker and Spiderman in different ways in the Ultimate version. And friendship is also likely to be a theme, as his relationships with his friends are important parts of his character.
There's 'Midnight Encounters'. This fanfic focuses on the more romantic side of their relationship. It shows how they gradually develop feelings for each other while still maintaining their individual identities as heroes/villains. It's a really sweet read.