To start off, most of your training should be in the 3-15 rep range (in my opinion). I recommend keeping your compounds on the heavier (1-7 reps) side and your isolations on the lighter (7-20 reps) side.
How often should you switch up your rep ranges?
Some say every workout, some say every week, some bro’s may say never (8-12 only bro ;)). Now most of us know if you never switch up your reps you’ll eventually plateau, especially once you’re past the “newbie gains” stage. On the contrary, if you change up your reps every workout you’ll have a hard time progressing and building off any singular number. This is why I recommend sticking with a certain number of reps (high or low) for a month or two before switching it up.
The last couple months I was doing (roughly) 5 sets of 6 reps on tricep pushdowns. Now I’m doing 2-3 sets of 20+ reps. This is just one example of how much you should be varying your training, with accessories especially. Now for compounds I would recommend having one higher rep (4-7) and one lower rep (1-3) day a week. For me this is on my first and last training day. This way you’ll be progressing with two different loads at the same damn time. Since implementing this strategy I’ve seen massive amounts of gains in both strength and muscle mass! In fact, focusing on strength has been one of the best things for improving my physique.