There is no real straightforward answer to this question, as usual, my reply to most subjects like this is “it all depends.” But for the sake of giving you something to work with typically training with reps of 6-12 are your best bet for packing on slabs of steak to your frame, this rep range should dominate a significant majority of your programming.
In saying that it is incredibly beneficial for you to occasionally go outside of this rep range and train with reps of 3 to 5 or even 20 to 30. If you only ever do one specific rep range your muscles will become desensitised to that stimulus. When we have a short phase in either the 3-5 rep range or the 20-30 rep range, we create a different incentive for the muscle. When we return to the ideal rep ranges of 6 -12 so too does the stimulus of that rep range, providing you bigger and better long-term results.
Here is an example of a 10-week program utilising such rep structures:
Week 1 – 4 – Rep range – 6/8
Week 5 – 6 – Rep range – 10/12
Week 7 -8 – Rep range – 3/5
Week 9 – 10 – Rep range – 20/30
Repeat