using more terrains until it looks smooth. notice that the terrains do not match exactly as I estimated, but if you keep adding terrains you might be able to get it smooth enough to fool the  eye.  note the green dot in the first pict. you will need to pull this up to the center point of your ring. the more terrains the further away the green dot needs to be. You will also need to turn the green dot on in the (A)tributes dialog by checking show origin handle. In this example I used terrains that were 1000x1000  and moved the dot up to 3000.  In multirep you will make 20 copies and 18 degrees x or z.(forget which).  This may help to calculate distance and number of terrains: 360 degrees to a circle (2 Pi radians) ... or not - good luck.

(used B5.5)