Straight up:
Anything from 5 to 30 reps, as long as it’s taken close to failure, will build almost identical amounts of muscle mass. But let’s get into the details, the nooks and crannies — the how and the why.
Think about it this way:
A heavy set of 5 reps is screaming muscle growth stimulus. But you’re only screaming it 5 times.
Now compare that to a set of 15 reps — you’re shouting muscle growth stimulus 15 times.
And then compare it again to a set of 25 reps, maybe something like a cable lateral raise — now you’re whispering muscle growth stimulus 25 times.
Shake that all up, put it into a cocktail, and what comes out is the same amount of “noise” — in this case, muscle growth stimulus.
So as long as you’re pushing close to failure, you’re going to get almost identical muscle growth from a set of 5, 15, or 25.
What’s great about this is that certain rep ranges are more conducive to certain exercises, sessions, or times of the week.
For example:
You might do heavy sets of weighted pull-ups or back squats for 5 reps. But you’re not going to do dumbbell lateral raises for sets of 5 — maybe 15 reps makes sense there. A set of 25 might suit dumbbell wrist curls, training around an injury, or slotting into a superset.
So don’t listen to the bullshit, don’t listen to the noise.
Anything between 5 and 30 reps — and there’s even some literature suggesting it goes as high as 50 (though let’s hold our tongue on that until we’ve got more evidence) — builds similar, almost identical amounts of muscle.
If you want to dive deeper into the research, google Brad Schoenfeld and rep ranges.