How do you know how many reps to do?

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.