Question

How many possible superkeys from n attributes where each attribute is candidate key

Answer 1

Any subset of attributes of a table that can uniquely identify all the tuples of that table is known as a Super key. Its different from the primary and candidate keys in the sense that only the minimal superkeys are the candidate/primary keys.

This means that from a super key when we remove all the attributes that are unnecessary for its uniqueness, only then it becomes a primary/candidate key. So, in essence, all primary/candidate keys are super keys but not all superkeys are primary/candidate keys. By the formal definition of a Relation(Table), we know that the tuples of a relation are all unique. So the set of all attributes itself is a super key.