Higher reps can build muscle and the body weight advance that goes with it. Higher reps can also build strength. Twelve reps can be very heavy, 20 reps can be even heavier if not seemly impossible for some. Higher reps work best with compound exercises.You can also call these extended sets. Take doing the DL for example
and your normal capacity is 400lbs for 10 good reps. Those 10 more reps to meet 20 reps will require a mind set to accomplish that. It will also require a few deep breaths between each rep from the start of the DL. This is a form of the Rest-Pause system where between 5 to 12 seconds (or more) are taken before the next rep is started again. At the last 3 or 4 reps you may be taking 8 or 9 breaths because of the intense nature of this style of heavy training.This is the same princple of the breathing squat program and it does work well if the effort is put into it. Than only 1 to 2 sets will be required for any compoumd exercise done in this style. That would be per muscle group. This should be the only case where you come near the point of failure The higher reps also work well when Im on a power rack cycle and doing heavy partials (3 to 5 inch movements) The good thing about higher reps is that the muscles are taking the blunt of the work and not so much the joints themselves, for me anyway. I also do heavy 3X3 in a power rack to encourage more strength to the joints for single efforts.
Counting for higher reps can play games with the mind sometimes, those 20 reps can seem impossible from time to time. Might want to try counting reps another way. Instead of counting the regular 1,2,3,4, etc until the 20th rep is reached, there is another way. Count 1, and ('And' as a rep), 2, and,3,and,4,and,etc, etc. When you get to the number 10 count that will be 19 rep and the final 'And' will be the 20th and last rep. So, in other words the mind connects with the numbers and not the 'And' so it belives that only 10 reps where done rather than the actual 20 reps. Tends to make higher reps a lot easier when the mind thinks it's only counting half of what is really done. An old time PL'er taughts me that once and it worked very well when I was doing very heavy breathing squats for 20 or more reps. Good Luck.