I am also still confused on the Array size expression you mention. I currently don't have an array in use, just a Json
Your JSON string actually contains arrays. "Scene" is an array, and "dialogues" is an array. If you see square brackets in JSON string - that's an array.
So JSON.ArraySize("Scene.0.dialogues") for example will return the number of elements in "dialogues" array in the first scene.
I suggest you read/watch some tutorials about JSON. It's not that complex and once you understand its structure, it will all make sense.