Japan’s first Nintendo 64 Pokémon Stadium game had 40 playable Pokémon.
The game, released in Japan as Pocket Monsters Stadium in 1998, was designed around 3D battles connected to the Game Boy Pokémon games. Although the original games contained 151 species, technical and design limitations restricted the number that could be used directly in battles to 40.
An in-game encyclopedia could still display information about all 151 species from the Game Boy games. The game also supported Nintendo 64 connectivity through the Transfer Pak, allowing compatible Game Boy Pokémon to be viewed, organized, and used in battles.
This title is easy to confuse with the internationally released Pokémon Stadium. The Japanese Pocket Monsters Stadium was the earlier game and was never released outside Japan. The international Pokémon Stadium was effectively a larger follow-up, with access to all 151 first-generation Pokémon.