A bit shorter...

=MAX(IF(ISNUMBER(A1:A20),A1:A20))

which you need to confirm to control+shift+enter instead of just with enter.

Paul wrote:
=Max(A1:A20) returns #N/A when one or more of the cells in
the series contains #N/A.

Is there a way to cause the formula to overlook the #N/A
and return the maximum value? Or, is there another
function that would accomplish this?