What happens if a Digimon with 10 or fewer cards stacked with it is chosen for this card's [On Play] [When Digivolving] effect?
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The top card is trashed until there are no more stacked cards. Once there is 1 card left, it isn't considered to have "stacked cards," therefore no more are trashed.
Source: BT21-030 Shoutmon X7: Superior Mode