After the words were written, a second, third, fourth and fifth scribe would check the work and each would write the exact number of words on the bottom of the page. The center word and character would be marked on each page as well.
The accuracy of the Bible had been questioned for years, until the discovery of The Dead Sea Scrolls. The oldest manuscripts were identical to those scrolls.
That’s for the Old Testament. Things were a bit looser in the Middle Ages otherwise.