Simplistically repetitions are dictated by your goals:
Reps Goal
1-3 Power
1-5 Strength
6-8 Functional Hypertrophy
8-12 Hypertrophy
12-15 Strength Endurance
15+ Endurance
In reality you have to consider many other factors, such as the persons training age, training experience, chronological age, sports participated in, body type, flexibility, nervous system, technical ability, past and present injuries, time restrains and demands, plus other lifestyle factors.
Also important to note that you should fatigue on the repetition range for your desired goal. If your goal is strength but you lift a weight that you can move two or three times more, then you will not be stressing the muscle enough to gain optimal response.
