I've always used the heaviest weight I can lift with good form and either shoot for 8 reps per set or I do pyramids starting with 8 reps 8-6-4-2-1-2-4-6-8.
If you're main goal is lifing for mass, what's the "lowest" useful number of reps you should use?
Conversely, what's the highest number you should ever be doing for a given set?