1 set. Personally, after a back workout, I can destroy my biceps in one set. Its painful but I'll share it with you here. I suppose you could use this when your biceps are fresh too, but you have to find the correct weight:

Single arm DB preacher curls:
*first, warmups with light weight for 6-8 reps, use "feeler sets" to find the appropriate weight (if you can performed 6 reps "easily" its too light for a working set)*

Working set is positive failure on ~3rd-4th rep.
Have a spotter help you perform 2-3 assisted reps.
Then have the spotter help you perform 2 resisted negatives (where they pull the DB downward and you resist them for as long as possible, throughout the entire negative ROM).
Then perform 2 slow regular negatives.

I haven't found a better bicep destroyer yet. Truly Brutal.