Ans is 2. {i.e. 2(2x1) MUX or 1(2x1) + 1 NOT}
Explanation : f= A'B'C + A'B'C' = A'B'(C+C') = A'B' = (A+B)' which is NOR so if we take A as a select line then the value of A either 0 or 1, if is A is 0 then output of nor gate is B' and if A is1 then output of nor gate is 0
Now input for the multiplexer is B' and 0 and select line is A then output is A'B' = (A+B)'