You should take all of the above factors into consideration when you are determining how many reps and sets you should do. Following a training program designed for your fitness level is the best way to choose how many reps and sets are appropriate for each exercise. A beginner may be crushing their goals by doing 3 sets of 5 reps for one exercise, whereas someone at an advanced level may be barely challenged at 5 sets of 10 reps of the same exercise. Focus on your personal milestones, follow a trainer’s guidance, and listen to your body!