If each exterior angle of a regular polygon is $18^{\circ }$, then the number of sides of the polygon is

The sum of all exterior angles of any regular polygon is always $360^{\circ }$.The formula relating the number of sides ($n$) and each exterior angle is:
$$n=\frac{360^{\circ }}{\text{Exterior Angle}}$$
$$n=\frac{360^{\circ }}{18^{\circ }}=20$$