@Oceana – Everyone is different, so I agree that it is best for the individual to decide how many reps is enough. I know that when I started doing my abdominal workout, there was no way I could have done the number of reps I can today, just as doing as few reps as I did then would have no effect for me now.
I started out doing fifteen crunches at a time. I would rest after that, and then I would do fifteen more reps.
Over several months, I slowly worked up the number of reps. I can now do a 100 rep workout program, and I do these fifty at a time.
I have hard, etched abdominals now. It took awhile to accomplish, but I did it the best way that I could, and that was by slowly increasing the number of reps.