For an 'n' variable function, total number of possible minterms(input combinations) will be $2^n$. Half of them will be one i.e, $2^{n-1}$.
Thus total number of neutral functions possble = Choosing any $2^{n-1}$ combinations to be 1 out of $2^n$ combination. i.e $2^{n} \choose 2^{n-1}$.