First, understand the goal of the exercise. What is the purpose? Will it concentrate on the goal? Once you have established the purpose, you will know what muscles are the focus. When fatigue sets in, that is the stopping point.
Ranges may be good for a starting point. In my opinion, to make the session truly personal, focus on the client’s form. Once the form begins to deviate from the concentrated muscles, the set is done. This will vary based on a few factors – rest, stress levels, orthopedic condition(s). These factors can change seasonally or per session.
A roundabout way to say, when the appropriate form for the individual alters, the set is done. The rep number will based on this. So, how many reps? However many can be done in good form.
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.
I agree with the general theme within most of the answers already given. It varies and depends on both client conditioning and goals. I would generally start beginners with lower weight, higher reps. On the other hand, experienced clients will lift heavier weight, less reps with respect to whether they want to grow muscle, build strength, or increase power. The links provided in above answers are good to identify your basic guidelines of reps and rest. Apply those and then alter as needed based on client conditioning and progress (or lack of).
To increase strength, there seems to be a consensus of opinion that 8-12 reps to temporary failure is quite effective. The degree of hypertrophy is individually, genetically, based. I’m not sure that I agree with Sebastian. The increase in myofibular protein must account for some degree of hypertrophy.
While lifting in general will lead to hypertrophy, recent studies have shown that lifting higher reps 10+ can lead to hypertrophy without necessarily strength because the endurance required more water to be retained in the muscles resulting in a larger appearance. Lifting within 3-6 reps drastically increases strength without necessarily muscle size.While the muscle fibers will grow, its the increase in water retention that really expands the muscles.