There are many opinions and many studies on what is the optimal number of repetitions and sets (set of repetitions) of an exercise to obtain the greatest benefits in the shortest possible time.
There is definitely a relationship with the genetics of each person that, in part, determines the ratio of fast muscle fibers (those that provide the main component of muscle strength) and slow (those that can perform work for longer periods).
Basically we will develop the skill that we use to train. If we train with a lot of weight, we will develop strength, if we do a lot of work (total weight lifted), we will improve our resistance.
For a focus on increasing strength:
In this case, it is best to do low repetitions and a lot of weight .
Example: From one to four sets of 3 to 8 repetitions.
For a focus on muscle volume:
In this case, it is best to do a lot of repetitions and a lot of weight .
Example: Three to four sets of 8-12 reps
For a focus on endurance (duration):
In this case, it is best to do high repetitions and low weight.
Example: One to ten sets of more than 25 reps
The numbers above are only a guide and results can vary considerably from person to person. Remember that it is very important to consult a doctor or a trainer before starting any physical exercise program.