Amazon has updated its page for the collectors’ edition of the illustrated Harry Potter and the Order of the Phoenix (by J.K. Rowling, with artwork by Jim Kay), and the listing says the book will be 576 pages long.
In comparison, Harry Potter and the Goblet of Fire is 464 pages long (and both editions, regular and collectors, are the same length). We are confident this number of pages will be final and the illustrated edition of the fifth book in the Harry Potter saga will have 576 pages. It will be interesting to see how they will accomodate all the text and illustrations in that amount of pages!