Graph > Contour Plot > OK > Contour Options > Levels
Use to control the number and placement of contour levels.
Contour Levels
Automatic: Choose to accept the default number of contour levels.
Number: Choose to specify the number of contour levels, then enter a number between 2 and 11.
Values: Choose to specify the z-values to plot as contours, then enter from 2 to 11 values.