It is unfortunately indicated anywhere you need 2 packages of 6 curved rails in order to build a complete circle. For an oval circuit, it takes 2 packs of 6 curved rails + 1 package of 6 straight tracks (over 30 in total just for the rails!).
It is totally useless to only order a packet 6 curved tracks (because you get only half a circle). Apparently, packs of 12 curved tracks do not exist ...
Nevertheless, the rails are high quality, easy to assemble (from 2 years) and very strong (even when some children walk on it ...)