Let's take 3 boolean variables for easy understanding .
a,b and c.
Each of the boolean variable can take 2 values , 0 and 1.
Thus total number of combinations possible for min terms = 2x2x2 = 8
And what are these? a'b'c' , a'b'c .... correct?
How to create boolean functions now ?
f(a,b,c) = Sum of min terms(you can also take Product of max terms , no issue :) ).
Each min term has two possibilities , either it can be in the function or not.
Thus total number of functions possible = $2^{Number of min terms}$ = $2^{2^{3}}$.
Similarly you can do it for n variables.