Summary:
In 1.20.5 pre-release 1 there was a change that removed the page limit in Written Books:
https://www.minecraft.net/article/minecraft-1-20-5-pre-release-1

The page limit in Written Books has been removed

In Bedrock this limit is still not removed and is 50 pages.

Steps to reproduce:

1. Open Written Book.
2. Try to create more than 50 pages.

Observed results:
You can only create 50 pages.

Expected results:
Written Books should not have a page limit