I also remember reading that the amount of highlighting allowed might be set by the publisher. I also remember reading, as Ann said, that the limit, when it exists, is on the percent of text highlighted, not the number of highlights. I have some books where I have a very large number of highlights of small amounts of text. For example, I have been highlighting the first instances of Names (people, locations, organizations, etc.) that I encounter while rereading the Discworld series. I have 246 highlights (plus 17 notes and 2 bookmarks) in
The Color of Magic. It did not stop letting me highlight the text. I do not know if this is because I did not hit the limit or if is a limit on this book. (Usually my number of notes far outweighs the number of highlights I have in a book.

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