didnt read all the replies but i tried 20 rep squats a year ago. i remember reading you should start it at 70% and then add weight each week
i had a 1RM of just over 140kg maybe, or 150
did it once a week. added 2.5kg each week
started at 105kg i think
after roughly 5-6 weeks i think i got to 117.5 for 20
week after that couldnt make 20 with 120kg
first 8 aint too bad. 9-10 getting hard. 12th you start struggling. after that its basically about getting in air. after 16 reps you're lucky to get 1 rep every 20-30seconds