. How many reps (repetitions) should I be doing per set of exercise ?
This depends greatly on your experience, the body part you are training and what your goals are. If you are a beginner or if your goal is to tone a muscle group without adding much mass to it, stick to performing sets with a higher number of reps and a lighter weight. Generally speaking, anything over 12 reps would (in most cases) be considered high-repetition training.
If your goal is to add mass to your body, perform sets in the six to eight rep range with moderate to heavy weights. This range forces your muscles to become fatigued quickly and using heavy weights will stress your muscles for maximum growth. (Warm up with one to two higher rep sets with lighter weights before using heavier weights in the six to eight rep range.)
If your goal is to strictly increase strength (as in power-lifting), most of your reps should stay between one and five using near-maximum to maximum poundage. Again, warm up with one or two higher rep sets with lighter weights before using heavier weights in this low rep-range. Remember that body parts such as calves and abs may require higher rep sets to become fully fatigued.