I love shrugs and I actualy have pretty good size and peaked traps, and truth is i have no idea, I just do set after set and go til failure with the reps so sometimes im sure i do maybe 4 sets but those sets each had 50 reps in them or more, or ill do maybe 5-6 sets and maybe just go super super heavy with maybe 20 reps per set,
I blast my traps and for me loads and loads of reps has always worked best for me, although i dont count my reps, i know i do anywhere from 20 reps super heavy per set or 40-50 reps medium heavy for a bunch of sets, My traps respond well to high reps