You don't need more then 10 torches, because you'll notice that the amount of torches in your bags will never get even close to zero. So there's not much gain in getting 40 torches.

I got the achievement eventually, by binding scroll to 1 and 2, the torches x 2. Then spam scroll and mouse button. Did require an empty dalaran to get it. around 23:15 game time i couldn't do it. Had an average of 28..... But next day around 16:00 i got it within a min.