Pokémon Pokopia's main Pokédex contained 300 Pokémon at launch.
Those 300 Pokémon were present in the game when the original version released. They form the numbered main Pokopia Pokédex and are directly available for players working toward Pokédex completion.
Later updates expanded the overall roster. Version 2.0.0 added 50 Pokémon alongside the Bubbly Basin downloadable content, while seven more were introduced through in-game events. As a result, 357 Pokémon were available in the game as of Version 2.0.0.
The numbers can be confusing because 300 describes the launch roster, while 357 describes the expanded total after later content. The main Pokédex figure also differs from the total number of Pokémon available through DLC and events. For a launch-focused question, the canonical number is 300.