However, it's safe to say that *millions* of people have read it, as Riordan's books are extremely popular.
Here's why:
* They're bestsellers: Riordan's books are consistently on bestseller lists and have been translated into numerous languages.
* They're popular with young adults: The target audience for these books is middle-grade and young adult readers, a demographic that enjoys fantasy and adventure stories.
* They're part of a series: The *Percy Jackson & The Olympians* series and the *Heroes of Olympus* series are interconnected, creating a large fanbase for each book in both series.
* They've been adapted into movies and TV shows: The Percy Jackson series has been adapted into movies and the *Heroes of Olympus* series is being adapted into a TV show, bringing even more attention to the books.
So, while we can't know exactly who has read *The Mark of Athena*, it's clear that the book has a large and dedicated readership.