For most people, there's little evidence to support the superiority of multi-set strength training regimen. In fact, a single set of 12 repetitions with the proper weight can build strength as effectively as can multiple sets of the same exercise. Simply use a weight that tires your muscles at 12 repetitions. As you get stronger, gradually increase the amount of weight. To help ensure continued progress in your strength training program, make your strength training workouts progressively more difficult. It also helps to vary the type of exercise you perform for each muscle group.
If you're hoping to increase muscle mass significantly — during bodybuilding, for example, or to enhance your performance in certain sports — adding additional sets may be beneficial. Remember, however, for most people there's no need to spend extra time in the gym doing multiple sets of the same strength training exercises.