The Splatoon manga is indeed canon. It contributes to the broader narrative and world-building of Splatoon. Any variations from other elements are usually intentional and part of the creative process to expand the story.
I don't have the specific number off the top of my head. Maybe you can ask manga experts or do some research on manga review platforms to get a clear answer.
I'm not sure exactly how many Splatoon manga books there are. You might want to check with specialized manga retailers or online databases for the most accurate count.
Good Splatoon manga fanfic often has vivid descriptions of the battles. For example, it can clearly show how inklings use their ink - based weapons. Also, unique character designs are important. The fanfic might introduce new inklings with different personalities and looks.
You can try looking on popular manga reading websites like MangaReader or Viz. Sometimes they have a wide collection of various manga, including Splatoon.