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?
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Official Ruling
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.
Source: BT15-021 Gomamon (X Antibody)