Graphic option
contour_levels
Default value: 5
contour_levels is the number of levels in contour plots.
Since this is a global graphics option, its position in the scene description does not matter.
Example:
(%i1) load(draw)$ (%i2) draw3d(explicit(20*exp(-x^2-y^2)-10,x,0,2,y,-3,3), contour_levels = 15, contour = both, surface_hide = true) $