For labeling only some of the lines in a contour plot, define a vector with the lines you want to label, and give that as an input argument to the "clabel" function.
As an example :
% Generate labels only for contour lines 2 and 6
[x,y,z] = peaks;
[C,h] = contour(x,y,z);
v = [2,6];
clabel(C,h,v);
