Well it depends on what you are trying to accomplish with the assistance work.
In the westside system the assistance work is there mostly to build muscle via smaller, more targeted exercises. So really, anything from 8 to 30 reps can be used (or even more). I personally find that I do better with higher reps for deltoid and pectorals (I go as high as 40 repos per set for pectorals but most of my assistance work for chest is done for 15-20 reps).
Hamstrings respond better to lower reps if using a knee flexion exercise (glute-ham raised, leg curl) ...so sets of 8-10 reps. For free-weight hip extension exercises (goodmorning, Romanian deadlift) I also like reps around 8, even 6. For hip extension on machines or pulley (reverse hyper, cable pull-through) I prefer sets of anywhere between 12 and 20 reps.
For upper back I focus ore on the quality of the contraction not so much n weight and the reps normally falls between 10 and 15.
For triceps and biceps I actually prefer shorter sets, 8-10 reps.