A common question I hear is ‘how many reps should I be doing?’. Let me start by stating the obvious: the purpose of muscle is to contract with force. Bear with me here.
Thus, a muscle will adapt to the demands you place on it. If you demand that it contracts with more force, it has no choice but to get bigger. Therefore, you should be training with the loads that ensure you can consistently demand that your muscles contract with more force each time you train. So the weight that you train with should be relatively heavy.
‘but the 8-12 rep range is best for size’. Yes, when you are handling heavy loads.
‘but the pro-bodybuilders always train with high reps and they’re massive’ - Again, they’re freakishly strong. There is a huge difference in muscle stimulation from lifting 40kg for 8 reps and lifting 180kg for 8 reps:
This, on the other hand, is not optimal for hypertrophy:
You must get strong before you can get big. Many pro-bodybuilders either started as competitive powerlifters or at least are very strong, e.g. Ronnie Coleman and Jonnie Jackson deadlift 362kg and 346kg respectively.
So the ideal rep range for most exercises at first should be under 5 while you aim to get stronger and increase your strength capacity. The important things are A) your form doesn’t break down, and B) you are not always grinding the reps. You will get strong, fast.
Only then can you increase the reps a little if you’re seeking more size, or keep them the same if you’re aiming for strength/speed.
However – that is not to say that anything over 5 reps won’t build strength, or anything under 5 reps won’t hypertrophy the muscle. You’re missing the point if you assign too much meaning to these numbers (which are just GUIDELINES). In all of your training, progression should be the emphasis. Whether you’re lifting more weight or more reps each session is the most important thing.