a exor b = ab' + a'b (when there are two inputs a & b, $2^{(2-1)}$ = 2 minterms
a exor b exor c = a'b'c + ab'c' + a'bc' + abc ( (when there are three inputs a, b & c, $2^{(3-1)}$ = 4 minterms
hence , with 8-inout EX-OR gate there will be $2^{(8-1)}$ = 128 minterms