I'm a big fan of cluster counting for higher rep sets. For 20+ reps, I'll usually count up for the first 10 or 15, then down in 5's for additional reps.
As to the original question, I'm quite sure most people can't squat their bodyweight for 20 reps. And I think Dan John's mentioned that number as a minimum benchmark for athletes. So to me, that's a number that, if you want to obtain and you're even reasonably coordinated/athletic, you should be able to, but it would be rare for an untrained individual to be capable of this.
For myself, I have no idea what weight would be my 'max' for this task, but I've done 345 for 5 sets of 10, with my 1-rep max in the mid 400's. I would feel comfortable hitting 20 reps at 315 any given day. I'm not suggesting this would be easy, but I'm confident that's a number I could hit. My morning bodyweight is right about 190.