The recommended number of situps for beginners depends upon your age, fitness level and situp form. Generally, men have more muscle than women, and younger individuals have more muscle than older individuals. With more muscle, you can do more repetitions. Top End Sports and other sources have promulgated charts which depict sit-up standards for various age ranges. As you gain experience and improvements in your fitness level, you could very well exceed these standards. Just as important as number of repetitions, however, is the form you use while doing situps. If you place your hands behind your head (versus across your head or on your thighs), interlace your fingers loosely and do not pull on your neck. The sit-up is only one kind of ab exercise. There are many.
Already Fit and Getting Fitter
If you are already relatively fit, but you want to score high on a military physical readiness test, begin with three sets of 25 situps on Monday and Friday, three sets of 15 incline situps on Wednesday, and on Saturday, three sets of 25 situps followed by a 10-minute rest and two sets of 15 inclines. Add five repetitions to each set each week for six weeks to reach your fitness goal, according to NavyPRT.com. "Inclines" are more commonly referred to as "declines" in mainstream gyms and a decline bench is used for the exercise. The Navy PRT does not actually require decline sit-ups or "curl-ups", but the decline variation is a challenge and will improve your performance on normal sit-ups.
Getting in Shape from the Very Beginning
If you are an older individual or have been away from exercise for a long time, you may benefit from partial situps, according to gerontologist, F. Michael Gloth, III. Lie on the floor with your hands behind your head and your knees flexed, in a situp position. But do not move your shoulders off the floor. For 20 repetitions, press your lower spine into the floor.