Eevee had eight official evolutions by Generation IX.
These forms are Vaporeon, Jolteon, Flareon, Espeon, Umbreon, Leafeon, Glaceon, and Sylveon. Together they are commonly called Eeveelutions, and they represent a wide range of Pokémon types.
Eevee first appeared in Generation I, where it could evolve into Vaporeon, Jolteon, or Flareon using Water, Thunder, or Fire Stones. Later generations added friendship-based, location-based, and move-based evolution methods.
Sylveon was the newest official Eevee evolution introduced by Generation VI. No new Eeveelution was added in Generation VII, VIII, or IX, which is why the total remained eight through Generation IX.