I am using multiple contour variables C1 C2 C3 and C4 in my plot but I can only see the legend for C1.
You must have the Contour Details dialog set to the desired contour variable (C1 C2 C3 C4) when you toggle on the Legend. And that contour variable must be in active use in the plot. If a variable is assigned to a Zone Layer like mesh coloring and you turn off the Zone Layer in the sidebar that legend will disappear. There are also limitations such as if the only use of a particular contour variable is to define iso-surfaces the legend will not be shown. But if that contour variable is used to color the iso-surfaces the legend will be shown.