What’s better for building muscle, low or high reps?
The answer isn’t quite that simple, and much like other controversial topics the answer falls somewhere in the middle. Due to differences in muscle fiber composition, different muscles hypertrophy (growth and increase of the size of muscle cells) best at different and often multiple rep ranges.
For maximum muscular development a variety of set and rep schemes should be employed.
In conclusion, hit multiple rep ranges (low, medium, high, super high) throughout the course of a week or training block. For most people this would be anywhere from 3-20 reps!