It doesn't matter if you do 2 sets of 10 reps, 5 sets of 4 reps or 20 sets of 1 rep — do whatever it takes to reach the 20 total reps goal. (fitnessandpower.com)
And one more related question: If I fail to reach the set / rep goals using a particular weight, I assume I should use that same weight again the next time I do that workout? (muscularstrength.com)
Make the exercise more difficult by changing up your foot positioning, and always strive to add more sets and reps. I also like to just shoot for a total rep goal — 100, 150, even 200 in a single session. (sixpackfactory.com)