Answer:
do them till you cant do them anymore, do them slow, and with correct form, and youll see results quicker than just flying through 100 crunches.
well it may help to tone ur stomach
It will tone up your stomach, but if you have fat over top the muscle you won't see the results. If you have fat over top start doing some cardio as well, walking, running, dancing etc.
Also look up some different types of crunches to do. That way your abs won't get used to the same move as easily. Once your body adapts, you'll have to do more to feel it. I do bicycle crunches, regular crunches, full sit ups, leg raises (lay flat lift legs straight up, and lower back down, but don't touch your heels to the floor). There are quite a few different types so try checking out some fitness websites and magazines.