Not many. When you are low and try to lvl in a lvl 70 map there is a huge penalty to prevent D2 style power lvling. That is why when power lvling, you have the low lvl character make the game so it is there lvl. Additionally, in T6 they will lvl so fast that the mobs are to low to be efficient. That is why this is such a good method. Each mob that spawns has their lvl set to that of the low lvl character and they keep updating as they spawn. You see similar effects in cursed chest events when you have to kill 100 mobs in the time given. Because of this, I found cursed chests to be the best way to power lvl and it took me about 10 minutes to power lvl 2 people with them.