The amount of sit ups that needs to be doone per day to achieve a six pack depends upon proper exercise form and diet, according to CalisthenicExercise.com. Sit ups alone do not build defined abs; it takes proper eating and cardiovascular exercise for fat loss, as well.
Muscle & Strength has an efficient beginner abdominal workout available online. Crunches focus on the abdominals more than sit ups, because they remove the hip flexors during the exercise. Adding a lower abdominal exercise, such as the leg raise, will work the whole abdominal section. More repetitions and exercises can be added as strength develops. Resting for two days between workouts helps reduce soreness, as explained on WebMD.