The pullup is a compound exercise that you can do almost anywhere -- you only need a sturdy bar to hold onto and you are good to go. However it is less simple to decide on the ideal number of pullups per session. Ultimately, how many pullups you should do depends on your own fitness goals.
Pullups mainly engage your lats, located on the sides of your chest. Your biceps, shoulders and core also get a good workout from pullups. Proper pullup form calls for grabbing the bar with a grip slightly wider than your shoulder width and making sure your elbows point at three and nine o'clock throughout the exercise. To do a full pullup, make sure you raise your chin just above the bar.
How many pullups you should do depends on whether you want to get stronger or develop more endurance. In an August 2002 study in the "European Journal of Applied Physiology," Gerson Campos and colleagues showed that people who did four sets of three to five reps gained more strength. People who performed two sets of 20 to 28 reps gained more endurance. So, lower amounts of pullups with a lot of weight will make you stronger, whereas higher numbers of pullups with less weight will allow you to do pullups longer.
Do sets with low amounts of pullups and high weight if you want to develop a stronger upper body. If you find it too easy to lift your body weight, you can increase the resistance by wearing a backpack with weights in it or strapping on a weight belt. In an article for "Iron" magazine, Sean Nalewanyi suggests five to seven as the ideal rep range to gain strength and muscle. If you use a simple pullup bar propped inside a doorway, make sure it is sturdy before adding too much extra weight. You may want to have a spotter nearby in case the bar comes loose.
Increase your endurance by doing as many pullups as you can. You may find it difficult to do more than a couple pullups when you start out. You can do half pullups to gain enough endurance to do the many full pullups needed to increase your endurance. For a half pullup, simply stand on a chair so your chin sits just above your pullup bar. Grab the bar as if you were in the up portion of a pullup. Let yourself hang for a moment and then slowly lower yourself.
It is good to think about your fitness on a daily basis. However, you do not need to do pullups every day. Two or three weight training sessions a week are enough for most people. If you want to work out most days of the week, consider splitting your routine into upper body and lower body days, to avoid working the same muscle groups too much.