How many gym badges does Ash Ketchum need to collect to enter the Kanto League in the original Pokémon anime?

How to get close to this number

The games are the answer key. This structure is inherited directly from the video games, where the same requirement gates the endgame, and it has been repeated in essentially every region since. If you have played any entry in the series, you already know the number without needing to have watched the anime.

The consistency is the interesting part. A structure established in one 1996 game has survived three decades and nine generations essentially unchanged, across games, anime and spin-offs. Very few design decisions in any medium prove that durable, and it is because the number is load-bearing: it paces a journey across a region into eight roughly equal chapters.

Note that the anime and the games diverge on plenty of details — which leaders appear, in which order, and what happens at the league itself — so a question about this franchise generally has to say which medium it means. Here the two agree, which is precisely why this particular number is the most reliable fact in the whole setting.