Building a competitive Digimon deck is where the game's skill ceiling actually lives, and it is a meaningfully different discipline from collecting. A larger deck is not a better deck, and in a game where memory is the scarce resource, the cards you include are mostly about which plays they enable rather than how many bodies they add.
Every competitive list is built around a named archetype with a specific game plan - an early aggro plan, a midrange plan, a control plan. The archetype determines which cards are worth including and, critically, what you are trying to achieve by which turn. A deck that does not have a coherent plan is a collection, and it will lose to one that does.
Standard Digimon deck construction uses a fixed deck size with a limited number of copies per card, and the exact numbers vary by format. What matters more than the specific figure is the principle: a tight list with few redundant cards draws its key pieces far more often, and in a game decided by tempo and memory efficiency, consistency beats raw power.
The common beginner mistake is including too many tech cards - single-use effects that sound powerful but dead-end your hand. Removing them is usually the single largest improvement a new player can make to a list.
Because the card pool keeps expanding with new sets, the competitive landscape shifts continuously - much faster than in the physical game, where a new set is a physical object with a print cycle. A list that dominates one week can be poor the next once the new cards redistribute power. Tracking which archetypes gain from each release is a genuine skill in the digital version.
As a free-to-play game, Alysion does not gate competitive play behind a purchase - but the economy of time and resources still applies. Players who cannot commit to the regular ranked and tournament cycle will find it difficult to keep a tuned list, so decide how seriously you want to compete before investing in the grind.