Keep writing and editing!
Actually, we call it a manuscript. It only becomes a book when it is published. But to answer the question, the amount of copy that gets cut all depends. Do you mean the edits the author makes or the edits the editor at the publishing house makes?
Most successful authors go through as many as twenty-five or fifty drafts before considering a work ready for publication. The amount of content that gets axed depends on the goals and experience of the author. Often even experienced authors will remove whole sections or chapters, or add or delete characters to make the story stronger. Even after the author is finished, the editor may make additional changes, hopefully smaller ones.
To minimize late-stage cutting, some writers make story outlines that include the intended plot points. That helps the story stay on track. Newer writers tend to “over write” a story, thus requiring many cuts before a work is ready for publication.
Almost no writer magically turns out a publishable story in the first round. Writing is like working with a lump of clay. The shape emerges under the artist’s skilled eye. That requires a lot of careful thought and work.