There isn't a set limit but its recommended to have over a thousand mostly so small youtubers don't apply. The only real requirement is to make a good quality PokeFind series.
https://docs.google.com/forms/d/e/1FAIpQLSeis9vGY970rXHqh6Kw4sHrKAybWcgJHVWgVqqEyrLzI2xoBg/viewform You can apply here.
