It really depends on the genre and the plot of your novel. Some might have just a few, while others could have quite a lot.
Well, there's no fixed number. It depends on what you need to flesh out the world and move the story along. Maybe five or six if they each have a distinct role, or maybe just one or two if they have a more minor impact.
The number of side characters in a novel varies greatly. It could be as few as two or three if the focus is on a core group, or it could be dozens if it's a sprawling epic with multiple subplots. It all comes down to how they contribute to the story's depth and complexity.