Based on multiple studies protein synthesis ("engine" for muscle hypertrophy) can be virtually triggered in any rep range as long as you hit contractile or technical failure on the exercise. (Intensity in the studies was ranging from 30% or 1RM all the way to 80-90% of 1RM)

This means you can trigger muscle growth in any rep range as long as you work to/or near failure.

In what rep range you should train?

In freakin' all rep ranges!

Lower rep range(1-5) is the way to put "mechanical tension" in the equation and to work on your maximal strength which is absolute prerequisite for muscle growth in the long run. It'll give you greater ability to progressively overload the muscle!

Moderate rep range(6-12) is a perfect balance between necessary intensity and time under tension which is why this rep range is also called "hypertrophy" rep range. Metabolic stress and muscle damage occurs here which are also keys for muscle hypertrophy.

Higher rep range(12-15+) is the rep range where metabolic fatigue massively kicks in. Usually higher rep range means increased time under tension which is ideal for increased cell volume/maximal pump which will create the perfect environment for nutrient drive into the muscle cells. Also you'll tap into "slow-twitch muscle fibers" in this rep range and although they have smaller growth potential opposed to fast-twitch they still add to the overall size and density of your muscles.

TRAIN IN ALL REP RANGES TO ENSURE YOU COVERED ALL MUSCLE BUILDING CONTRIBUTING FACTORS!