Please pay attention to directions:
20 rep sets have long been used to build the widest possible base for training
See article here: https://barbend.com/20-rep-squat-workout/
When warming up for a singular 20 rep set try and build to a weight that you can comfortably do for sets of 8 to 10. Then you want to do that weight for 1 VERY HARD set of 20.
The key is VERY LONG ranges of motion. Do not cut your reps short.
AS AN EXAMPLE: If you squat 225 for 15 reps and fail on the 16th. Then on the following week you should go for at least 16 reps with 225. If you squat 225 for 20 reps the following week you should try to squat 235 for 20 reps.
This training plan is as hard as you make it. Good luck!