Inosuke doesn't become a Hashira in the manga. The story progresses with him having other significant experiences and achievements instead of achieving that specific position.
It's hard to give an exact number. Different story arcs and battles led to the deaths of multiple hashira. Some died heroically, while others met their fate in unexpected ways. The count changes as the manga progresses.
Definitely not. Genya remains outside the ranks of Hashira throughout the manga. This is because of a combination of factors such as his skills, experiences, and the overall direction of the story.
Well, it could be about a BTS member accidentally running into a gymnast YN at a sports event. Maybe they get to know each other gradually and BTS is inspired by YN's dedication in gymnastics. It could also involve YN getting a chance to perform at one of BTS's concerts, creating a unique and exciting plot.