Whenever an ability says 'once during your turn', that means it can be used once per Pokemon you have in play with that ability. For example, if you have 2 Delphox XY in play, that means you can use Mystical Fire twice per turn.
On the other hand, certain abilities have text that specifies that they can only be used a set amount of times per turn, such as Plasma Metagross & Victini NVI.
On the other hand, certain abilities have text that specifies that they can only be used a set amount of times per turn, such as Plasma Metagross & Victini NVI.