The original Super Smash Bros. for Nintendo 64 has 12 playable characters. Eight are available from the beginning, while four—Luigi, Captain Falcon, Ness, and Jigglypuff—are unlocked by meeting specific in-game conditions.
The roster was small by later Smash standards, but it represented several major Nintendo franchises. Mario and Luigi came from the Mario series; Donkey Kong represented his own arcade lineage; Link came from The Legend of Zelda; Samus came from Metroid; and Fox came from Star Fox. Pikachu and Jigglypuff brought Pokémon into the crossover.
The number 12 refers to distinct playable fighters in the completed roster, not the number visible immediately on the character-select screen. That distinction explains why some players remember eight. It is also easy to confuse the original roster with later games: Melee expanded the cast substantially, while Ultimate eventually grew to dozens of fighters through its base roster and downloadable content.