In Elden Ring, the highest upgrade level for a standard weapon is +25.
Standard weapons are improved with Smithing Stones, beginning with ordinary Smithing Stone [1] and ending with an Ancient Dragon Smithing Stone. Their upgrade path runs from the unmodified weapon at +0 through +25. Each group of Smithing Stones covers several upgrade levels, while the final level requires the rare Ancient Dragon material.
This system is separate from the one used by special weapons. Unique weapons, including many boss and legendary armaments, use Somber Smithing Stones and stop at +10 instead. A +10 unique weapon is therefore fully upgraded even though its number is lower than +25.
Upgrade level is not the same as weapon rarity, reinforcement affinity, or the character’s attributes. A standard weapon can also be changed with Ashes of War, which may alter its scaling and damage type without changing the maximum reinforcement level.