@TDahl makes a good point. The minimum number of segments can be calculated with a formula, but there’s a simple plugin to do that for you. For a 120mm radius, it gives this:
The " Tolerance" value indicates that the maximum deviation from a true circle to the flat is no more than 0.025mm (which is well below the tolerance of the 3D printer). The suggested value of 156 will include the four cardinal points.
The plugin can be downloaded from here:
https://sites.google.com/site/spirixcode/code/jrh_n_calc.rbz