The number of reps you should do when working out depends on what the goals are that you have set for yourself. What results do you want? Are you wanting to gain strength? Anaerobic strength endurance! Aerobic strength endurance? If strength is the goal, lower reps (3-8) would be what you want to do. 10-20 reps will yield anaerobic strength endurance and high reps (20 and above) will yield aerobic strength endurance. A regular person wanting to add a little muscle mass (aka toning and definition) would want to fall in the medium range. I recommend 10-15 reps. If you can comfortably do 15 reps its time to increase the amount of weight you are doing. The last 3-5 reps should be difficult but you should be able to use proper form.