[ContourLegendFrequency(frequency)] controls how many (if any) legend entries are skipped on a contour plot. If set to 0 or 1, every label is drawn. If set to 2, every other label is skipped. If set to 3, every 3rd label is drawn, etc.
There is currently no way to control the placement or the maximum number of rows in the legend for 3D data, sorry. It is on my to-do list.