I believe he means to disregard the amount of sets it takes.
Shoot for maximal reps at each set until you hit the target number. He suggests you try for at least 20 reps on the first set.
Thus, your ab work could look like this:
1st set - 22, rest a few seconds
2nd set - 15, rest a few seconds (total reps 37)
3rd set - 10, rest a few seconds (total reps 47, must do 1 more set to hit 50)
4th set - 7, total reps are 54, done with that exercise
If it takes you more sets so be it, and if it takes you less, that is fine too.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano