How do the professors know when the Pokedex is complete?
If they ask you to complete the Pokedex, how will anybody in-universe even know it's complete? My only guess is that maybe the professors can analyze when the Pokedex has no further storage-space left for new entries. Give me your ideas (or better yet, if there's somehow official info, inform us).
Answers
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Presumably they know how many Pokémon there are in their region, you are collecting data on them, not discovering them for the first time.
Mythical Pokémon could be presumed to be new discoveries.
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They're professors so they know how many Pokémon have been discovered (in general and in their region), they don't think you actually found all Pokémon, just all the ones you could.
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They know already all the existing species, since Mythicals are called that way because they are heard of but their existence was never confirmed (never seen by most). Legendaries are confirmed to exist and tied to legends, and the rest of the pokémon in a region are seen commonly.
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