So, it's less about a concrete word count, more about getting the chance to see how he emotionally reacts to meaningful situations (whether outwardly, internally or both). Generally, the more variety and nuance you can supply, the better.

I'd say not to use a prompt that what-ifs what would have happened if something prior to his canon departure happened differently, as that would still put you in the position of effectively producing a sample for a character that isn't the one you're applying for. But you can explore canon moments, small moments in between canon scenes (ex: Pikachu stubbed his paw-toe-thing that AM, Ash dealt with the emergency, how does he process that in the afternoon?), something of your design, or you're welcome to use one of the props from our latest event.