We could do our own experiment right here. Max on bench and refer to a rep Max chart to find the correlating weight done for 20 reps. Try that and see if it works... Or use Jim's favorite equation.
Weight x reps x. 0333 + weight = estimated max
If you know your max and want to determine a different variable such as reps for a given weight required to reach your max then:
Reps = Max - weight / weight /. 0333
Weight + weight= max / reps /. 0333
If Max = 300 and you want to do 20 reps then this would look like:
Weight + weight = 300 / 20 /. 0333
Weight + weight= 450
Weight = 225
Man, I just spaced out in nerd mode for a bit there...