Forty Pokémon could battle in the Japanese Nintendo 64 game Pocket Monsters Stadium.
Released in Japan on August 1, 1998, Pocket Monsters Stadium was the first Pokémon Stadium title. It brought Pokémon battles into 3D and connected with the Game Boy games through the Nintendo 64 Transfer Pak.
Technical limitations reduced the battle roster from the full 151 Pokémon planned for the Game Boy generation to 40. The selected Pokémon were chosen largely for their use in official tournaments, with additional choices helping represent the different types.
The later international Pokémon Stadium was not simply the same release with a translated title. It expanded the battle roster to all 151 original Pokémon and included additional features. This difference explains why sources sometimes describe Stadium as supporting either 40 or 151 Pokémon.