You might think that more repetitions equals more muscle, but that is not the case. The number of repetitions you perform each set will have a big impact on whether the primary effect of your workout is developing muscle strength, size or endurance. Low repetitions in the one-to-six range are best for building strength between 7 & 12 reps is best for adding muscle mass and 13 or more repetitions will develop muscle endurance.
These are, however, broad guides and are on a spectrum rather than self-contained blocks. Performing three or six repetitions of an exercise, for example, will build strength but doing 6 reps will have more of a size development effect than performing three, because it's closer to the size gain range of the spectrum.
FAIL TO SUCCEED :
In each case, to get the desired effect, you should aim to reach failure (when you can't complete another repetition without compromising form) at your target repetition count on the final set of the workout. If you complete your repetition and feel that you could perform more, you are not using a heavy enough weight.
It's also important to remember that these repetition ranges are general guides. Not everyone responds to resistance training in exactly the same way and even different muscles in the body can respond different, depending on their function. For example, slow-twitch muscles fibers (the smaller muscle fibers involved in long distance endurance efforts) will generally experience strength gains at a higher repetition range than fast-twitch muscle fibers (the larger muscle fibers involved in short, explosive movements).
Your level of training experience will also play a part in your results. Generally, people new to weight training will develop strength into a slightly higher repetition range than more experienced workouts.
The number of repetitions you perform will affect your results.
Effect Repetitions
Strength 1-6
Muscle Mass 7-12
Endurance 13+
