DB Heroes doesn't follow the manga exactly. It takes inspiration and builds on it, making changes to suit the specific requirements of the series. However, the core elements are still recognizable.
The length of DB mangas differs. It depends on factors like the story's complexity and the author's vision. Some might run for just a few volumes, while others could have dozens.
I'm not sure specifically what 'db new story' is about without more context. It could be a new story related to a database (db), perhaps a new narrative within a database - related project, like a new data - entry story or a new development in a database system's history.