The 500,000 number is the number of books that are part of the settlement. If they downloaded all of Libgen and the other sources it was more like >7Million. But it is a lot of work to determine which books can legitimately be part of the lawsuit. For example, if any of the books in the download weren't copyright (think self published) or not protected under US copyright law (maybe a book only published in Venezula) or it isn't clear who own the copyright then that copyright owner cannot be part of the class. So it seems like the 500,000 number is basically the smaller number of books for which the lawyers for the plaintiff felt they could most easily prove standing.