Each player sacrifices two creatures.
5/1/2012 The active player chooses which creatures will be sacrificed first, then each other player in turn order does the same. Then all creatures are sacrificed simultaneously.
5/1/2012 If a player controls only one creature, that creature is sacrificed.
5/1/2012 Barter in Blood doesn’t target any creatures and may be cast even if a player controls fewer than two creatures.