Mixed
Resistance training protocols utilizing moderate repetition ranges (approximately 6-12 reps) optimize muscle hypertrophy by maximizing metabolic stress, whereas high-repetition training is inferior unless artificial ischemia is applied.
To maximize muscle growth, aim for sets of 6-12 repetitions. This range balances mechanical tension with metabolic stress, which is crucial for hypertrophy. Avoid very high repetitions (15+) unless you are using special techniques like blood flow restriction, as they are generally less effective for building mass.
The use of high repetitions has generally proven to be inferior to moderate and lower repetition ranges in eliciting increases in muscle hypertrophy (24,71). In the absence of artificially induced ischemia (i.e., occlusion training), a load less than approximately 65% of 1RM is not considered sufficient to promote substantial hypertrophy (115).
Why this rating
The paper is a comprehensive review citing multiple studies, though it acknowledges debate between low and moderate reps.
Source
The Mechanisms of Muscle Hypertrophy and Their Application to Resistance Training
Brad J. Schöenfeld · The Journal of Strength and Conditioning Research · 2010
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