the answer must be a difinition just like what guix suggested also
I did change it to
#define numA 300
#define numB 600
#define numC 100
#define numD 200
#define Max(x,y) ((x)>(y) ? (x) : (y))
#define Nummax Max((numA),Max((numB),Max((numC),(numD))))
and it worked!
Is still possible to shorten it even further to make the sketch looks cleaner?