Who's number one?
Number One backpack is a Epic Fortnite Backpack. This back bling is currently unreleased. It's an upcoming backbling that have been leaked but hasn't been put on sale in the Fortnite Item Shop yet!
The backbling Number One belongs to Chapter 2 Season 3. This backbling is a leaked backbling, an upcoming item that will release in the Fortnite Battle Royale game. Therefore, you cannot buy it yet at the Fortnite Shop or get it by any other method.