That is one set.
For unilateral (only doing one side at once) you count the number of sets on each side. So you do 12 on left, 12 on right, and then you've done "1 set". Another 12 on left and another 12 on right is "2 sets".
Sets is about how much each muscle worked. It would be wrong to say you did "2 sets" for 12 on left and 12 on right, as each muscle only moved 12 times, not 24 times.