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 depends. Just because the story is based on a real story doesn't necessarily mean the serpent is a literal, physical snake. It could be a symbolic serpent representing something in the real - life events, like a person or an organization that acts in a sneaky or dangerous way, much like how a snake is often seen in literature. But if it is a more straightforward account, then it could be a real snake.