For XOR gate, if the no. of 1s in the input is even then it results in 0 otherwise 1.
eg- 11010100 will result in 0 whereas 11010101 will give 1
So, possible inputs from 8 bits are 28 =256
Minterms are those which result in 1 or those combinations which give 1 OR Minterms = Total possible inputs(28) - Combinations of inputs which give 0.
Inputs which result in 0 value have 0,2,4,6,8 number of 1s = 8C0 + 8C2 + 8C4 + 8C6 + 8C8 = 128
Minterms = 256-128 =128