It depends what your goals are, but if you want to build strong abdominal muscles you will have to aim for 12-25 reps (weighted if needed). This range can change, depending the exercise you're doing and how your body react to it. But the main goal is to perform all the movements in a slow and controlled manner.
If you are not feeling pain around the 10th rep, you probably need to add more weight, or you're doing it wrong.