The aim is to find a formula for the question.
For $n=2$ i get $2^{2^n}=16$ possible functions.
This is the solution for a boolean function with 2 attributes:
01) a -> separable
02) b -> separable
03) not a -> separable
04) not b -> separable
05) a and b -> separable
06) a or b -> separable
07) a xor b -> not separable
08) a nand b -> separable
09) a nor b -> separable
10) a xnor b -> not separable
11) (not a) and b -> separable
12) a and (not b) -> separable
13) (not a) or b -> separable
14) a or (not b) -> separable
15) (not a) xor b -> not separable
16) a xor (not b) -> not separable
But what is the formular for n?