It's hard to give an exact number. The series has been ongoing for some time, and new volumes may be released. You could search on dedicated book sites or ask in related online communities for the latest count.
High School DxD's story is considered good by many. It offers a unique blend of genres and has a decent amount of depth. The character interactions and world-building add to its appeal. But, as with any story, it depends on personal preferences.
Asta could be brought into the High School DxD fanfic as a result of a multiverse - related event. In this case, different universes are colliding or being connected in some way. His presence would disrupt the power balance as his anti - magic is something that no one in the High School DxD world has really encountered before. He could end up training with the devils to learn more about their magic while also teaching them about the concept of anti - magic.
One way could be through the power system. The EMS (Eternal Mangekyou Sharingan) from Naruto could be a unique power that a character in the High School DxD world acquires. Maybe a transfer student from a hidden ninja village arrives at the high school and has to keep their EMS a secret while dealing with the devils and angels in the school. This could lead to some interesting battles and interactions as the powers of the EMS, like the ability to use powerful jutsus, contrast with the magic and abilities of the High School DxD characters.
In High School DxD fanfiction, the Juggernaut is often portrayed as an extremely powerful being. It might have a menacing appearance with its large and imposing physique. It could be shown as having a single - minded focus on achieving its goals, whether it's destruction or some other objective.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.