Erin, two reps of 15 is all you 'really' need if you do them right - if you do them 'right' you shouldnt be able to do any more than that. If you do more you're just fooling yourself.

Getting down on the ground or on a weight bench and going through the motions isnt doing your ab muscles any good - they must be contracted for every move. The motion of the crunch is very small - you raise your shoulders off the ground and that it. But you do this while squeezing your abs, forcing the breath from your diaphram and squeezing your glutes (which isolates the abs and takes the hip flexors out of the picture) When done right you'll feel those 30 crunches the next day.

Nat