You can choose any number of your opponent's Digimon, as long as the play costs of the chosen Digimon add up to 15 or less. For example, you could choose 1 Digimon with a play cost of 3 and 1 Digimon with a play cost of 12, or 1 Digimon with a play cost of 2 and 2 Digimon with play costs of 6.

Yes. As long as the total play costs of the chosen cards is less than 15, you can choose as many or as few cards as you like. However, note that you must choose a minimum of 1 Digimon with a play cost of 15 or less.

No. Regardless of how a Digimon was played, only the play cost printed on the card is counted for this card’s effect.