For n independent boolean variables, each taking one particular boolean value, there are 2n different possible combinations. A boolean function has to assign one boolean value to each one of these combinations. This brings the number of different possible boolean functions of n variables to a total of 22n When n=1 we have only one boolean variable that can take either boolean value, so we have only 21=2 different cases. This produces 22=4 boolean functions of one boolean variable.