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None.
It has no vertices as such but it does have a side that is called its circumference which has 360 degrees around it.
Vertices are on a shape when two sides meet up and create a corner. For example, a square has 4 vertices, a tringle has 3 vertices, a circle has no vertices. (singular is 'vertex').
A circle with any radius, including zero.
Half an octagon has five sides is it is divided in two by a line joining opposite vertices. It has six sides in the dividing line joins the midpoints of opposite sides.