Its pointless to worry about this. Choose a total rep volume say 20. Then do your sets paying attention to your speed. If you do 15s then keep tension high by controlling the weight with slow tempo but as you slow down stop. This is to avoid too much neural fatigue. Now with that said volume is dependent on your connective tissue resistance, meaning the more experienced you are the more volume you may need to stimulate hypertrophy. Keep in mind that volume also depletes glycogen which the body prioritizes before protein synthesis. The point is stimulate with enough tension to activate your satellite cells. So worrying about how many sets is not based on the principles. You must understand whats going on phsyiologicaly to get the most out of training. Read the FAQ. As long as you increase the load for at least 6 workouts, 9 would be my suggestion, (Read HST summurized by Lyle) then you will keep protein synthesis high. Also frequency determines volume. See I do 15 reps 5 days a week, regardless if im on 15, 10 or 5s. Its about experimenting and figuring your body out. Now ... learn, eat and grow.