Fact: More muscle has been grown in the 8-12 rep range than all other rep ranges put together. If your goal is primarily muscle growth, that's your sweet spot.
Although the bulk of the current scientific research on muscle hypertrophy strongly suggests that growth is directly correlated with how many "hard" sets you perform per muscle/per week, that same research also argues that the number of reps per set doesn't really matter much, as long as those sets are taken to, or at least near, momentary muscular failure.
So you could indeed build tons of muscle using sets of 2-3, or, conversely, sets of 20-25. In fact, many lifters have done just that. In practical terms, however, there are some very good reasons why you shouldn't veer toward either extreme.
Low rep (1-7) training
Requires much heavier weights, which comes with disadvantages:
- Greater risk of injury.
- Greater sympathetic nervous system ("fight or flight") stress.
- More warm-up sets, which lengthens overall workout duration.
- More time between work sets, which also lengthens the time you'll need to spend in the gym.
High rep (13-plus) sets
These come with only one primary drawback, but it's a significant one: Assuming that the value of a set is hinged upon taking it to failure, it becomes clear that only the last few reps are responsible for the benefit you receive from the set as a whole.
If you do a set of 20, those first 16-18 reps only serve to get you to those last few result-producing reps. They're necessary, but also a waste of time and energy. But if you do a set of 8, you'll only need 5-6 reps to get you into the growth zone of that set.
The Benefits of (Occasionally) Using Extreme Rep Ranges
Given those drawbacks, there are a few different legitimate reasons to (at least sometimes) do either low- or high-rep training:
- Low-rep sets have a much greater impact on strength development. Even if you don't care about being strong, getting stronger will help you use more weight for your sets of 8-12 down the road. Or, it might simply be that you like being strong. If that's the case, definitely do a fair bit of low-rep work.
- High-rep training has more cardiovascular impact than lower-rep work, and it also burns more calories (probably not per unit of time, but because high-rep sets keep you working for longer durations). Also, even though high rep sets require more reps in order to get close to failure, you won't need as many warm-up sets because you'll be using less weight.
Finally, there's one additional reason to stray from the 8-12 range: If you always, or nearly always, train in that range, your body has likely habituated to that specific stressor and is likely reacting less strongly to it than it used to.
If this is the case, try venturing into the 5-7 range for a month or two, followed by another 4-8 weeks in the 15-20 range. You'll be pleasantly surprised by the results.