The optimal number of reps for hypertrophy-oriented training is a source of ongoing debate in the fitness field. Although the research is by no means conclusive, evidence indicates that a moderate rep range (about 6-12 reps per set) is best for maximizing muscle growth.
This is often referred to as "bodybuilding-style training" as it provides the ideal combination of mechanical tension, muscle damage, and metabolic stress – the three primary factors involved in hypertrophic gains. The problem is, most lifters seem to think this means all training should be carried out in this rep range and thus they rigidly adhere to the same loading patterns. Wrong assumption.
Maximal muscular development is built on a foundation of strength. This mandates that at least some of your sets need to be carried out in the lower rep ranges (1-5 reps per set).
Stronger muscles allow you to use heavier weights, and thus generate greater muscular tension in the moderate repetition ranges that optimally stimulate hypertrophy. By increasing muscle tension without compromising metabolic stress, you're setting the stage for enhanced growth.
On the other end of the spectrum, high rep sets (in the range of 15 to 20 reps per set) also have a place in a hypertrophy-oriented routine. Provided that you train at or near your sub-rep max, lower intensity sets help to increase your lactate threshold, the point at which lactic acid rapidly begins to accumulate in working muscles.
The problem with lactic acid is that beyond a certain point its accumulation interferes with muscle contraction, reducing the number of reps you can perform. Technical note: It's actually the H+ component of lactic acid that hastens the onset of muscular fatigue. Not lactic acid itself.
Here's the good news: Higher rep training increases capillary density and improves muscle buffering capacity, both of which help to delay lactic buildup. The upshot is, you're able to maintain a greater time under tension at a given hypertrophy-oriented workload. In addition, you develop a greater tolerance for higher volumes of work – an important component for maximizing hypertrophy.
Optimal muscle development is achieved by varying your rep range over time. This is best carried out in a structured, periodized program. Both undulating and linear periodized approaches can work, depending on your goals. Whatever scheme you employ, though, make sure you include the full spectrum of loading ranges.
Sure, hypertrophy training is probably best achieved with moderate-rep sets, but higher and lower intensities are nevertheless important for optimizing muscular development.