The Bubbly Basin DLC added 50 Pokémon to Pokémon Pokopia.
Bubbly Basin was released as part of the game’s version 2.0.0 content. The expansion increased the roster beyond the 300 Pokémon available when Pokopia first launched, adding 50 new Pokémon associated with the downloadable-content update.
The addition is part of a broader post-launch expansion of Pokopia’s world. The same version also included other new content, while seven Pokémon were introduced through in-game events rather than through the Bubbly Basin roster itself.
That distinction matters when counting the full Pokopia Pokédex. Adding 50 to the original 300 gives 350, but including the seven event Pokémon results in 357 available Pokémon as of version 2.0.0. Therefore, 50 refers specifically to Bubbly Basin’s contribution, not the entire post-launch increase.