I know we rag on Joe Weider from time to time on here but when I bought my first ever DBs they came with a 3x per week full-body workout. Annoyingly the workout required a barbell but I felt reassured seeing Weider's name plastered along the top and joined a gym. The workout had me working 2 sets of 6 reps 3x per week. I even had a slightly different set of 14 exercises each day for a total of 42 exercises.
So, thanks to Mighty Joe, I started with a six-rep scheme and saw that it worked wonders. After a couple of months the program had me doing sets of 5 and then a 9-rep backoff and that 5-6 rep-range has stuck with me ever since like a staple.