Programs are written in sets and reps, but the number that often matters most is one you will not see on the page: how close each set came to the point where you could not complete another rep with good form. A set of ten that could easily have been twenty does far less than a set of ten that could have been twelve.
Rep ranges: wider than you think
There is good evidence that muscle growth can occur across a broad range, from roughly 5 to 30 reps per set, as long as the sets are hard. Very light loads work, but they need many reps and become uncomfortable in ways that have little to do with the muscle.
Strength is a little more specific. If your goal is to lift heavier loads, some of your training should use heavier weights for fewer reps—say 3–6. For most people, a mix works well: heavier work on the main lifts and 8–15 reps on supporting exercises.
Reps in reserve
Reps in reserve, often written RIR, is a simple question you ask at the end of a set: how many more good reps could I have done? If you finished 10 reps on a row and could have managed two more cleanly, that set was at 2 RIR.
For most training, 1–3 RIR is a sensible target. It is close enough to failure to drive adaptation, and far enough from it that technique holds and recovery stays manageable. Going to complete failure occasionally is fine, especially on safer exercises like machines or curls, but it does not need to be routine.
Beginners tend to underestimate how many reps they have left, sometimes by a lot. That improves with practice. One way to calibrate is to take a final set of a safe exercise to failure now and then, and compare it with what you predicted.
How many sets
For most people, somewhere around 10–20 hard sets per muscle group per week produces good results, and beginners often progress well on less. More is not automatically better; beyond a point, extra sets mostly add fatigue. Individual tolerance varies widely, so treat any number as a starting point.
What to do this week
After each working set, note the reps and your best estimate of reps in reserve beside them—for example, '135 lb x 8, 2 RIR'. If most sets are landing at 4 or more, add 5 lb or a couple of reps next time. If they are routinely at 0, hold the load and tidy up the reps.
Part of