max(a, b) returns the maximum of the two values a and b. It only accepts two arguments. Getting the maximum of six input signals is a bit cumbersome:
max(u, max(u, max(u, max(u, max(u, u)))))
The same applies to min(a, b).
'u' only references the first input signal and is the same as u(1) or u.
You could also use intrinsic blocks (max, min, sum and gain) to achieve the desired functionality.