Well, there's no fixed rule. A novel could be a single book if the story is concise and self-contained. But if the plot is expansive and involves multiple subplots or character arcs, it might stretch into two, three, or more books. It all comes down to what the author wants to convey and how much space is needed to do it justice.
It really depends. Some novels can be just one book, while others might be part of a series that could have three, five, or even more books. There's no set number.