TextStep controls which contours are labeled.
Consider this example.
x = -2:0.2:2;
y = -2:0.2:3;
[X,Y] = meshgrid(x,y);
Z = X.*exp(-X.^2-Y.^2);
[~, C] = contour(X,Y,Z,'ShowText','on');
Note that the contour labels are [.1, .2, .3, ...] (image below, left)
Look at the current TextStep value.
Double the value
Note the change in text labels. (image below, right)
A common approach is to set TextStep as a factor of LevelStep.
C.TextStep = C.LevelStep*2;