The boss fights in the Nephalem Rifts often drop multiple blood shards. They show up in bright purple and it says "Blood Shards (X)" where 'x' is the number I received from that fight. It seems like every time I fight one of these rift bosses, I get a different number of blood shards. And, when I play with some of my friends, they usually get a different number than I do. What determines how many blood shards are dropped from Nephalem Rift boss fights?