Consider we have n attributes represented as a bit. We can put 1 when an attribute has to be a part of superkey or 0 otherwise. So this will be (2^n), because we have two choices (1 or 0) for each of the n bits/attributes. We subtract 1 to avoid all 0's, that is considering 'no-attribute' as a superkey. So (2^n)-1 superkey are there.
