Each leg has to do the total number of reps for that set to complete the set. So you would be right if you said "right leg front then left leg front as one rep". Now, when doing side step ups, say your first set is a set of 12reps. That means that you would step up to the side on the box 12 times with your right leg and then turn around and step up to the side 12 times with the left leg (or vice versa depending on which leg you want to start with). This would complete your first SET of 12reps. You would then repeat this process for the remainder of your sets for this exercise.