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.
In my view, 'fiction db' is probably a term used in a particular field or industry related to fiction. It could be a database used by authors, publishers, or researchers to organize and access data about fictional works. But again, we'd need more details to know for sure.