What are their goals? that’s my first question before I make any suggestions on weights and reps. And that assumes no underlying injury which would modify the programming.
Lifting for power and bulk (not my clientele) tends to be fewer repetitions at weights closer to their “one rep max.” Few lifts, heavy weight, long rest.
Lifting for strength improvement without bulk, which is the majority of my clients – 8-10 reps of a weight that is challenging but do-able with proper form.
Lifting for endurance or returning from injury – light weight, 15 or more reps
Personally, I try to be in that ballpark with my clients, but I don’t obsess over whether they did 8 reps or 9. My key cue for them is to find out whether they feel like they could do a bunch of reps (too easy) or if they feel like they can’t safely get through a half-dozen.
as Nancy said, the number of repetitions depends on what the client’s goals are.
Having said that, if I start with a client who is a novice to exercise, I tend to gravitate towards a lighter weight for at least 10 to 12 reps with just a little fatigue. With older clients I am even more cautious when they do not have a history of exercise.
For rep ranges there really is No one right answer, but yes it does depend on goals. Allow me to explain. If you had a power lifter, then yes they would work in low rep ranges, with high weight on their main lifts, but higher reps on other lifts. And if you had a strength lifter then they would work in the 3-5 range on the core lifts, and higher reps elsewhere… and then there is everyone else. And as recent evidence from Brad Schoenfeld and others has conclusively shown, one can develop hypertrophy regardless of the rep range so long as the muscle is exerted to near failure for several sets. Ergo it is muscle fatigue that is the key, not the rep range. So to play is conservative and safe I have clients work in a range of 8-12 in the beginning to develop better form. lastly, reps do no dictate muscle size, or bulk.. eating does. You can train with low or high reps all you want but if you dont enough, or too little no change will happen.
I agree that the first thing you want to take into consideration is the client’ goals with regard to rep range (as others have already mentioned). Whatever rep range you choose, you want the weights to be heavy enough to overload and fatigue the muscle. Whatever rep range you choose, the client should be pushing those last few reps–and really not be able to do any more of the total reps in good form.
Hope this helps…