With the intermediate workouts, Monday is 20 total reps per exercise, Wednesday and Friday are both 40 total reps per exercise.
You’re not using a light weight on Monday. Like the manual says, you’re using a 4-5 rep max which is pretty heavy. You’re also using only 30 seconds rest between sets (for the first week, less in later weeks), so you just keep plugging along until you hit the total reps for the exercise. It might take 5 sets or it might be 8 sets.
This article talks more about that type of training. It’s a different mindset from the usual “X sets and Y reps” but very effective at keeping the intensity high throughout the workout: