How many sit ups should a 12 year old female do?

For my school athletics, I do the following crunches:

30 crunches, right elbow to left knee

30 crunches, left elbow to right knee

30 crunches, legs bent to the left

30 crunches, legs bent to the right.

That adds up to 120 crunches. You could do many more, or even less. It really depends on how fit you are.