According to Muscle & Fitness, if a person is trying to build muscle, he should do eight sets of three repetitions. If he is trying to tone muscle, he should do three sets of eight repetitions.
As a general rule, more sets with fewer repetitions builds muscle while fewer sets with higher repetitions tones muscle. Body type and goals also play an important role in determining the appropriate number of sets per workout session, notes BodyBuilding.com. Serious athletes might do three sets of eight repetitions, while beginners do three sets of 12 to 20 repetitions. An exerciser should choose an amount of weight that allows him to perform the desired number of repetitions and sets without overexerting himself.