Well,
Take a triangle. It has three vertices and three sides. The three vertices dictate that there will be three sides. The same rule applies for a quadrilateral (four vertices and sides), a pentagon (five vertices and sides), etc. So the number of vertices in a polygon is equal to the number of sides in the same polygon.
Taking this a step further, since the number of vertices are equal to the number of sides, we can conclude that a polygon with three vertices is a triangle, a polygon with four vertices is a quadrilateral, a polygon with five vertices is a pentagon, etc.