more then one. less then all of the reps.
i have seen people grow with as few as 5 reps and as many as 20 rep sets. with a total of 25 reps a week upwards of 250+ reps per week. what you need take into account along side of reps is intensity and frequency. throw in sleep (quality and quantity) and food intake and IMO you have a loaded question.
here is a link from another thread on here:
Muscle Fiber Activation and Rep Range pt 1
here is an image of a chart that explains the various rep ranges and what they do:
and to confuse you even more: