in its mana cost, X is considered to be 0.Archmage's Charm offers three distinct jobs for one blue-heavy slot: counter a spell, draw two cards, or take a nonland permanent with mana value one or less. Holding three blue mana open can represent an answer while still allowing you to draw cards if no urgent threat appears.
The control-changing mode is more versatile than its small number suggests. Many ordinary creature tokens have mana value zero, so a large token can be a valid target even if its power is enormous. A one-mana creature or inexpensive artifact may also be worth taking. Token copies can retain a copied card's mana value, however, so not every token qualifies automatically.
In control and tempo decks, the draw mode turns unused defensive mana into progress. You can wait through an opponent's important decisions, then draw two near the end of the turn if nothing needs countering. The card does not require choosing the draw target as yourself, although doing so is the usual value play. The restrictive cost is the main deck-building consideration. Three lands that produce some combination of colors may still fail to cast it. A heavily blue mana base is more comfortable supporting Charm alongside other early interaction. When choosing between its modes, compare the resources gained: taking a useful permanent can remove an opposing threat and add one to your board, sometimes producing a stronger exchange than simply drawing two replacement cards.
