Text (OD+errata): When a player casts a spell, sacrifice ~this~. If you do, each of that player's opponents draws three cards. [Oracle 2009/07/08]
It only works once. If someone plays another spell after it triggers, but before that trigger resolves, then it will trigger again. The first time one of these triggers resolves, it will be sacrificed for the full effect. Any additional triggers on the stack will do nothing when they resolve because you will be unable to sacrifice it additional times. [Odyssey FAQ 2001/10/04]
