If the digivolution cards under the target opponent's Digimon increase to become more than those under this Digimon after this card's inherited effect activates, can my opponent's Digimon now attack?

No, it can't. If the number of digivolution cards under the target opponent's Digimon is less than or equal to those of the Digimon with this card in its digivolution cards at the timing when this effect activates, the target Digimon can't attack until the effect ends at the end of your opponent's turn, even if the number of its digivolution cards later increases.

Gomamon (X Antibody) rulings (BT15-021)

1 official ruling from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

Effect: [On Play] [When Digivolving] Reveal the top 3 cards of your deck. Add 1 card with the [Sea Beast], [Plesiosaur], [Beastkin] or [X Antibody] trait among them to the hand. Return the rest to the bottom of the deck.
Inherited effect: [When Attacking] [Once Per Turn] 1 of your opponent's Digimon with as many or fewer digivolution cards as this Digimon can't attack until the end of their turn.

Official Q&A

If the digivolution cards under the target opponent's Digimon increase to become more than those under this Digimon after this card's inherited effect activates, can my opponent's Digimon now attack?
No, it can't. If the number of digivolution cards under the target opponent's Digimon is less than or equal to those of the Digimon with this card in its digivolution cards at the timing when this effect activates, the target Digimon can't attack until the effect ends at the end of your opponent's turn, even if the number of its digivolution cards later increases.
Ruled Mar 28, 2024

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Gomamon (X Antibody) rulings (BT15-021)

1 official ruling from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

Effect: [On Play] [When Digivolving] Reveal the top 3 cards of your deck. Add 1 card with the [Sea Beast], [Plesiosaur], [Beastkin] or [X Antibody] trait among them to the hand. Return the rest to the bottom of the deck.
Inherited effect: [When Attacking] [Once Per Turn] 1 of your opponent's Digimon with as many or fewer digivolution cards as this Digimon can't attack until the end of their turn.

Official Q&A

If the digivolution cards under the target opponent's Digimon increase to become more than those under this Digimon after this card's inherited effect activates, can my opponent's Digimon now attack?
No, it can't. If the number of digivolution cards under the target opponent's Digimon is less than or equal to those of the Digimon with this card in its digivolution cards at the timing when this effect activates, the target Digimon can't attack until the effect ends at the end of your opponent's turn, even if the number of its digivolution cards later increases.
Ruled Mar 28, 2024

Got a line that uses Gomamon (X Antibody)? Map it in the builder and share it with a link.