User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-3146930-20121005000339/@comment-3146930-20121006165650

When they are talking about the prophecy at the start of the book (before Leo blows up stuff), Annabeth mentions that the prophecy requires 7. She then identifies Leo, Piper, Jason, and herself as four and Percy says that Hazel, Frank, and himself are three more.