In general, if we know a candidate key then the number of possible superkeys are $2^{n-1}$ , where n= total no. of attributes in a relation.
Let me explain through a simple example...
i) Consider Relation R=(A,B,C,D) where A is a candidate key. Find the total number of possible superkeys?
Total no. of super keys = $2^{n-1}$ , where n= 4
= $2^{4-1}$
= $2^{3}$ = 8
ii) Let a Relation R have attributes {a1, a2, a3,…,an} and the candidate key is “a1 a2 a3” then the possible number of super keys?
Following the previous formula, we have 3 attributes instead of one. So, here the number of possible superkey is $2^{n-3}$.