Answer is quite simple..
they have asked number of candidate keys "possible"... so we can assume that they are interested in maximum number of candidate keys..
Now definition of candidate key is: CK is a single field or least combination of fields that uniquely defines a row.
So possible number of combinations are same as possible number of binary strings.
n + nC2 + nC3 + .. + nCn
For this we have to consider maximum number of combinations with same number of attributes.
for example.. if we have five attributes(A,B,C,D,E) then if we take individual attributes we can not take any other. But if we focus on AB, BC.. then we can consider all combinations with 2 attributes but not with 3 or 4. In this way we are getting maximum candidate keys.
If there are 6 attributes then we will get maximum CK at 6C3..
In general we can say, nCfloor(n/2) will give maximum combinations that we can consider as CK..