First of all, how deep are you intending to make it? Other than a few extremely large species, it probably needn't be more than one meter deep. Perhaps two for those larger individuals. And consider that even a 3 by 3 pit would actually have more than sufficient surface area for up to nine Pokémon of approximately human size. So while a substantially smaller pit would require them to take turns, it certainly wouldn't need to be one at a time.
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