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.