Eevee has eight different official evolutions.
The first three were Vaporeon, Jolteon, and Flareon, introduced alongside Eevee in Pokémon Red and Green. Later games added Espeon, Umbreon, Leafeon, Glaceon, and Sylveon.
Eevee’s unstable genetic code is the fictional explanation for its branching evolution paths. The methods vary: some evolutions use elemental stones, while others depend on friendship, location, time of day, or a special move.
Eevee is sometimes described as having eight forms rather than eight evolutions, but the standard term is Eeveelutions. Pikachu and Pichu are related through an evolution line, yet neither has anything close to Eevee’s eight-way branching structure.