If two creature cards are placed in the graveyard by one card effect, will Sidisi, Brood Tyrant make one or two tokens?
2 Answers
"Whenever one or more creature cards (...)" only triggers once even when you mill multiple creature cards at the same time, like when Sidisi enters the battlefield.
If it was meant to trigger for each individual card, it would say: "Whenever a creature card (...)".
See one of Sidisi, Brood Tyrant's rulings:
9/20/2014: Any instruction, including Sidisi’s first ability, that tells you to put multiple cards from a library into a graveyard moves all the cards at the same time. For example, if Sidisi enters the battlefield and you put three creature cards into your graveyard, Sidisi’s last ability will trigger once and you’ll put one Zombie token onto the battlefield.
Bear in mind, one effect can have multiple separate instructions to move cards to your library: in general, if it contains multiple sentences, each with its own self-milling instruction, each sentence is a different instruction, and each instruction will move all of its cards simultaneously.
The current standard format doesn't seem to contain any examples of this, but imagine a Dampen Thought targeting yourself, spliced into another Dampen Thought, also targeting yourself. You'd wind up with, effectively, a spell saying: "Target player mills four cards. Target player mills four cards." — which will count as milling yourself twice, and trigger Sidisi twice.
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Worth noting that in other cases, just because an e.g. token-creating ability triggers only once, doesn't mean it only creates one token. Some "whenever one or more..." abilities will later say "that many", meaning the number of things that caused the trigger. (Or "that much", in the case of gaining or losing life, dealing or preventing damage, etc.) Commented Nov 25, 2023 at 2:16
Whenever one or more creature cards are put into your graveyard from your library simultaneously, Sidsi's second ability will trigger once, and thus produce one token.
603.2c An ability triggers only once each time its trigger event occurs. [...]
An instruction to move multiple cards moves the cards simultaneously.
Any instruction, including Sidisi’s first ability, that tells you to put multiple cards from a library into a graveyard moves all the cards at the same time. For example, if Sidisi enters the battlefield and you put three creature cards into your graveyard, Sidisi’s last ability will trigger once and you’ll put one Zombie token onto the battlefield.
But you asked about effects. It depends on the effect.
If an effect reads, "Move the top two cards of your library into your graveyard", Sidsi's ability will trigger at most one time since it only contains one instruction to move the cards.
If the effect reads, "Move the top card of your library into your graveyard. Move the top card of your library into your graveyard", Sidsi's ability would trigger twice, since the cards aren't moved simultaneously.