How many fin's should a rocket have?

It depends on a lot of stuffs like how much do you want your rocket to be stabile. But at least it must be 3. For example for the same stability you may have 3 fins that have large surface area or 4 which is smaller or more. When you increase the number of fins with the same surface area then the drag will increase because of the surface friction so 3 or 4 fins are generally used but there is no certain limit of the number of fins .