When I'm writing, the number of pages depends on the text size that I use. I've looked at some pdf books that I have and many of them seem to be size 16, so that's what I prefer to use, although I personally prefer 12.
When I look at physical books that I have, the number of words per page is about 400 or so.
Kindle books, well, the number of pages depends on what text size you set it at. I set it at the smallest size and it seems to be on average about 200 to 250 words per page.
So when I think about how many pages my book is going to have, I really don't know. It stands to be 30-40 pages, but that's based on size 12 text in Libreoffice.
How do you figure out what a page is?