Elden Ring has three main ending achievements: Elden Lord, Age of the Stars, and Lord of Frenzied Flame.
These are the three endings recognized by separate achievement or trophy entries. The Elden Lord ending includes several selectable variations tied to different Mending Runes, but those variations do not create separate achievement categories.
The Age of the Stars ending is associated with Ranni’s questline and sends the world into a new age under the guidance of the stars. The Lord of Frenzied Flame ending follows the Three Fingers and the influence of the Frenzied Flame. The remaining achievement category covers the Elden Lord outcomes.
A common mix-up is counting every narrative variation as a separate achievement. The game contains more than three possible final states when all Mending Rune outcomes are considered, but the achievement list groups them. This question therefore refers specifically to main ending achievements, not every ending variation.