Yev33 is correct, it's never the sets but the amount of total reps for any given muscle area. Sets only divided up the reps into workable goals. That 24-36 total rep range seems to get the most bang for the buck..for general BB'ing purposes of gaining strength and muscle.
Can include successful square root programs like 5X5's (25 reps) and the much overlooked 6X6's (36 reps) into that mix.
The other main square root programs ,10X10's (GVT) and 8X8's use a much higher rep scheme, but will use one main compound exercise for any one muscle area. The same moderate weight is usually throughout all the total set, the weight is never adjusted for any set.
Through experience, we all find that 20 reps may do for you what 5 reps can't and vice versa. Good Luck.