I got all reindeers after 26 bosses. I had to use 10 gems(potion) for boss 26 though. I like this event, sort of like a 2 in 1 kind of thing. Thank you GREE!!
Prancer - 3
Cupid - 4
Comet - 9
Dancer - 2
Blitzen - 1
Dasher - 1
Vixen - 3
Donner - 1
Rudolph - 2