There isn't really a limit to the number of pages you can have in Book Creator online. In Book Creator for iPad we coded the limit to be 9,999 pages.
But the truth is, the number of pages isn't the thing to look out for - you should be considering the overall file size of the book. The more pages, the bigger your book will be.
Obviously a single blank page won't add up to much, but every time you add media to your book (particularly large images or video files), that will exponentially increase the size of your book.
This is important for two reasons:
Large media files in your book may take longer to open when viewing them online (depending on the speed of your internet connection).
If you download the book to share it, the ePub file may be too large to send.
These days, it's probably not possible to create book so big that it can't be shared with a cloud app. Older versions of the iPad used to fail with files that were bigger than 250mb - you may still have issues with files beyond that size.
How do I know the file size of my book?
In Book Creator online, the only way to find this out is to download the book as an ePub. The file browser on your computer will tell you how big the downloaded file is.
In Book Creator for iPad, you can tap on the title of your book on the bookshelf and see the current size of the book (listed under iBook file size).