Are you talking the exercise; Crunches? If so then it is the weight you will be using to do the Crunches.

The exercise is misleading. If you are not using any weight then enter "0". For many that do Crunches they will use a weight plate to aid in creating resistance. I used to use a 50 lb weight and do them. Here is what I did and what I would suggest.

Put a weight (that you are comfortable with) on your chest and do 3 sets of 10. Make sure hold your abs tight at all times and when you lift up hold a few seconds and then go down … but don’t release the abs. To make it harder you can increase the amount of time you hold it at the top … currently I’m doing 30 seconds. Intensity/Quality is better then quantity.

When I do these I sometimes turn white; almost to the point of an ill feeling; but it passes. When done it is like opening a grill cheese sandwich; I am done. I have found nothing better for working abs that you can do anywhere and anytime; and doesn't cost anything.

Hope this helps.

Thanks for your answer.
It's interesting and I think I will try with weight on my chest.

Excellent. The whole thing with weight lifting is resistance. Adding more and more reps is not the way to increase size and strength; resistance is the only means. For crunches theoretically once could end up do hundreds if not thousands of crunches but to what end? Add resistance through weight and add resistance through the clenching of the abs and holding them throughout the lift, you will see far better results. The same thing goes for all exercises. If you could bench 200 lbs would you continue to just add more and more reps? Of course not, instead you would start to increase the weight as it got easier; crunches are the same thing. All muscles need to be treated the same way, resistance.

Yes, I read about the reps.

I find a good beginner routines with 4 sets of 12-15 reps, when easy, I will add weight