This challenge sounds much easier than it actually is. There was once a time that I thought crunches were some of the easiest things in the world, then, my ballet teacher taught me how to complete a real crunch. I was able to complete three. The next day I could not believe how sore my abs were from such a simple thing.
To do a proper crunch, begin with your back on the ground in sit-up position. Hands should be behind the head and the lower back should be resting on the ground. Your first crunch will be a little different from all the others since this is the only one where the upper back will be touching the ground... To complete them, only use the contraction of the abdominal muscles to lift the torso off the ground.
Once the torso is off the ground, keep the abs contracted for a few moments, then slowly lower so everything below the shoulder blades is on the ground and everything above the shoulder blades is still lifted. Then,start the whole process over again keeping the upper body off the ground. If you can do ten, you are doing quite well! If you do more than five and are not quite feeling it, you may need to re-evaluate what muscles are being worked and how the crunch could be tweaked.
So, how many can you do?