Hello Betsy, I have never heard of any kind of limit on the number of pages in a book. However, once a book gets too big it’s harder to find a good binding to hold it together, and it’s harder to pick the book up. If you’re thinking of anything longer than maybe 3000 pages, you should talk with a printer about proper binding, good thin paper, etc. to make it usable. And consider dividing it all into separate volumes.

You’ll be gathering several InDesign documents into one Book file to create a book. If you’re just getting started on your chapters, it’s best to design one of your more complex chapters first, then use that chapter as a template for all the remaining ones (i.e., “Save As” Chapter 1, delete all the original chapter’s text, place the Chapter 1 text, and thereby leave the trim size, margins, and styles intact and consistent among all chapters).

Warning  Making even minor changes to an outline numbering scheme won't necessarily change the initial position you've selected in the gallery, but rather may create a new gallery position, overwriting an existing one. Because of this problem, attaching numbering to styles is strongly recommended. This is covered in greater detail later in this chapter and in the chapter on Styles.
My quick process involves setting up a row of plaques with a01, a02, a03, thru a09. Then copy the plaques to the clipboard. Run the REPLACE TEXT feature to replace "a" with "1". Move row down and paste clipboard back into page. Run REPLACE TEXT to replace "a" with "2". Repeat until I get all the numbers. It's faster than manually typing the numbers in but I know there's go to be someway of automating the process. document numbering in publisher