The reps or number of repetitions mean the number of times you do a complete exercise movement during a set. A contraction followed by an extension to the point in which you need to start the movement over again, to repeat it.
The number of reps you can squeeze in a set depends greatly on the amount of weight you use for the exercise. Typically it is recommended that one use around 70 % of his/her maximum one rep capability. This will allow a person to execute around 6-12 repetitions for the upper body muscles and 12-16 repetitions for the legs.
The legs can do more repetitions because of the fact that they have far more endurance than the upper body muscles.
Legs are being used all day everyday, this makes them the strongest of body-parts, and the ones with the most endurance. Just imagine how tough our arms would be if they had to carry around all day something equivalent to our own bodyweight, Of course there are sets performed that feature a greater number of reps with smaller weights - for definition and fat-burning, but also for a warmup set before starting with the actual sets - or only a small number of reps with massive weight for maximum mass-gain and strength.