Surley it goes on the number of "lines" the shape is made up of, a triangle has 3, a square has 4 and a true circle is made of 1 single line therefore has 1 line?
A circle has one side. (One curved side).
(The joke about one inside and one outside does not count because this only relates to the line that is used to draw the shape. If the line is thick enough there will be no inside as the circle will be completely filled).
A semi-circle has two sides, one curved and one straight.
A quadrant has three, two straight and one curved.
and so on....