What page does Snape ask them to turn to while covering for Lupin's class?

394 is the page that Snape ask them to turn to while covering for Remus Lupin’s class as temporary Defence Against the Dark Arts professor. Severus Snape rushed into the class and closing the windows behind him, he orders the students to turn their books to page 394.

Harry Potter asks why Professor Remus Lupin who normally teaches the class isn’t teaching the class that day. Severus Snape answers by saying that isn’t his concern and reminds Harry to open page 394. Snape instructed the students to write an Essay on werewolves.