consider we have 2 boolean variable a and b.--> n variables
They can have 4 diff combinations ab,ab',a'b,a'b'.--> 2^n combinations possible
example of boolean function can be ab+ab' or a'b or ab+ab'+a'b' and so on.
Now to be part of a function each combination have 2 options.--->each of 2^n combinations have 2 options
Therefore total functions are 2*2*2*2.
I hope my approach is correct.