Pokémon Pokopia requires 27 Mysterious Slates to unlock Mew.
Mew is obtained through a special puzzle rather than a normal habitat encounter. Players collect 27 different Mysterious Slates from treasure spots throughout the game, then take them to ruins in the Withered Wastelands.
The slates must be arranged correctly to complete a mural. Their backs identify the Unown positions they belong to, while the completed arrangement forms an image associated with Mew. Once the mural puzzle is solved, Mew appears and can be befriended.
This method distinguishes Mew from several Legendary Pokémon in Pokopia, which use other requirements such as bells, special buildings, or story progression. The number 27 refers specifically to the different slate pieces in the Mysterious Mural puzzle, not to Mew’s National Pokédex number or the number of Pokémon in the game.