it depends were you are at in your training right now. i go through 'cycles' of training so that i can optimize growth, strength, and explosiveness. i will start by doing a 4-6 week hypertrophy phase consisting of roughly 20 total sets, with all exercises in the 8-12 rep range. Then i will do a strength phase that i will go down to around 16-18 or so sets, in the 5-8 rep range for 4 weeks. Following that i do a power phase for 2-4 weeks, 10-12 total sets, in the 2-5 rep range.
By approaching training by phases, your able to not only gain mass, but actually develop better mind muscle connections and strength. The reason for doing these phases in this order is:
-hypertrophy phase adds more actin fibers and myosin heads to the internal structure of your muscle, building up the proteins=building muscle size.
-strength phase increases the strength and functionality of the newly built muscle proteins.
-power phase sets a new higher level neural response to your muscle contractions, recruiting all your muscle fibers for a lift will enhance the bodies neural responce...
Its important to incorporate all of these phases and rep ranges into your training in order to maintain functional mass accumulation.
But to answer your question in short; depending on experience go with an 8-10 rep range for all exercises for no longer than 6 weeks straight. failure at the last rep every set and you will most certainly put on some mass.