You should do the number of sets and reps that align with your fitness goals. This is debatable in the fitness community, but for strength and toning, you should aim at hitting between 10 - 15 reps with a challenging resistance. Endurance training should be more like 25 - 50 reps. This is a question that needs much more explanation as there are many more variables that need to be considered. This question most definitely warrants its own article.