I have encountered paginated articles that reloaded the whole page every time I changed the page and it was bad UX, long articles with too much to scroll and no navigation options.
So I would consider making sure that the user can easily navigate to the section he is interested in, be able to search through it and other UX elements.
If pagination is the way to go I would consider these two things: Ease of use, navigation etc. and consider the regular phisical book character count which I found to be:
" In rough terms, each page of a standard-format hardcover book has about 300-350 words, and each word is five characters plus a space. So a typical book page has, say 1,500 to 1,800 characters (not counting spaces.)."