A box of six makes a half curve, 180 degrees. 

C Track Curved Track, Radius R1 = 360.0 mm / 14-3/16 inches
Every piece is 30°.

You will need 6 pieces for a half circle, and 12 pieces for a full circle.

These are the standard length pieces for a R1 radius curve.
If you need 1/2 the length, use a 24115 (that would be 15 degrees)

Also sold as a single piece under Marklin 24130.

Charlie's Tip: If you would like to lay two curves, side by side, the next larger size is the 24230 (R2 curves).

To go larger from there, you use R3 (Marklin 24330), or R4 (Marklin 24430) and finally R5 (Marklin 24530).

Read Charlie's blog on C-track curves