This question typically depends on what the client is trying to get out of their workout. Do they specifically want to build muscle mass or work on endurance training for that particular body part. Typically when an individual is looking to build endurance the repetition is around 12-15. Conversely if an individual is trying to build mass they will typically use a higher weight with lower reps. This is usually around 5-10. The repetitions that a individual does of an exercise is dependent on a lot of factors. To start, the repetitions in a exercise regimentation should reflect the goals of the client and the outcome that the trainer is trying to achieve in that specific mesocycle or macrocycle accordingly. You must also ask yourself what are we trying to achieve in this specific workout. All of these considerations should determine the type of regimentation and reflect the end goal that you and your client are striving for in the end.