Research has revealed that reaching momentary muscular failure within a certain range of repetitions can influence the results of weight training. Below is a chart that you can use to get the results you want from resistance training.
*Results from different research studies can have varying outcomes of +/-2 repetitions. I have included the entire span of these ranges. It is up to you to find the specific range that your body best responds to. If you find that 12-20 reps is not putting on as much muscle tone as you desire then try increasing the weight and lowering your repetitions to 5-8 reps.