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FOUR ghosts. Marley isn’t a footnote; he’s the catalyst. His death defines Scrooge’s life, his ghost initiates the moral reckoning, and his chains provide the moral framework of the story. The three later spirits simply show what Marley has already told: change or perish. (Personal obsession)

If we want to be really specific, it’s one ghost and three spirits. The spirits aren’t the shades of dead people, they’re supernatural entities. Marley and the ghosts Scrooge sees out the window are the only actual ghosts.