There are 710 unique items in Median XL.
Each regular item has one unique version. The stats of each unique depend on the tier of the base item: higher tier versions of the same unique are improved in all aspects, but have higher requirements.
The Sacred tier of each item has two unique versions that are different from the tiered unique. Sacred uniques can only drop in Destruction difficulty. The best rings, amulets, jewels and quivers are also marked as sacred uniques.