To build a full circle you need 32 pcs.
The R120 is the radius measured from the center of the circle to the centerline between the rails using the Lego stud as the unit. Lego stud is 8mm. The space needed to have the full R120 circle can be calculated as follows: (120 + 4) * 2 * 8mm = 1984mm = 1.98m. This measures the diameter (2 * radius) of the outer shape of the track - that's why we added 4 studs in the formula.
In general, the formula for calculating the outer diameter of the track circle is:
(R + 4) * 2 * 8mm