Dark Magician Girl

Per-phrase breakdown

• Gains 300 ATK for every "Dark Magician" or "Magician of Black Chaos" in the GYs. (Continuous)

Judge-facing facts

• Normal Summon / Set from hand uses 1 tribute (Level 6).
• The ATK increase is a Continuous Effect.
• The ATK increase updates immediately if the number of 'Dark Magician' or 'Magician of Black Chaos' in either player's GY changes.
• Not treated as a 'Magician' archetype card.

Card-specific notes from database

• (Konami neuron) The ATK boost is a Continuous Effect that applies while face-up in the Monster Zone and updates immediately when the GY contents change.
• (YGOResources) If an effect requires a monster originally named 'Dark Magician Girl', it can still be activated even if her current name was changed by another card effect.
• (Yugipedia wiki — verify) Not treated as a 'Magician' archetype card due to its Japanese name ('Black Magician Girl').