You go get your dumbbells and begin preparing for your workout, but then you stop and ask yourself, "Wait, how many reps am I supposed to do?" It's quite a tricky question to say the least. Looking online doesn't really help and asking people at the gym gets you all kinds of answers.
The truth is, the amount of reps you do will depend on your goals. Resistance training goals are typically based on building strength, building muscle, or increasing muscle endurance. Each of these goals relies on different rep ranges.
The science behind rep ranges consistently show that performing between one to five repetitions is best for building strength. This range stimulates a more efficient neuromuscular drive that increases signal firing rates from the brain to the muscle to improve contraction. This range also stimulates myofibrillar hypertrophy, which develops muscle by increasing the amount of force-generating components in the fiber known as actin and myosin, which then leads to stronger muscles. Going at a quick tempo, such as a 1 second down and 1 second up cadence, and resting at least 3 minutes between sets is recommended at this range.
As we progress into the six to twelve rep range, we shift the focus to muscle growth. This range also stimulates myofibrillar hypertrophy but not to its maximum potential. What makes this range effective in muscle growth is that it theoretically stimulates another type of hypertrophy known as sarcoplasmic hypertrophy. This fuels the muscle cell with more cellular fluid, glycogen (aka stored carbs), and more organelles. More of these means a larger, but not technically stronger, muscle. This explains why bodybuilders are larger but typically weaker than professional weightlifters. Performing a slow and controlled tempo, such as 3 seconds down and 2 seconds up, with no more than 90 seconds rest between sets is recommended in this range.
If you're looking for more endurance in your training, high reps are the name of the game. This range can be anywhere between 15 to upwards a hundred reps, and will typically stress out your energy system rather than the muscle itself. Doing so will allow the body to become more energy efficient by adapting to the metabolic stress. High rep, endurance training might not help you lift heavier, but you will be able to lift lighter weights for much longer while using less energy. Much like a gas saving hybrid car, it can't produce a lot of power at once, but it'll be able to work much longer while using a lot less energy. A moderate tempo such as 1 second down and 2 seconds up with no more than 90 seconds rest between sets is best for this range.
When choosing the correct rep range, make sure to choose an appropriate weight. If you feel like you can do at least 3 more at the end of the set, go heavier, and if you can't finish all the reps in the set, definitely go lighter.