As of Version 12.0, all the functionality of the Polytopes package is built into the Wolfram System. »

Polytopes Package

This package contains functions that give geometrical characteristics of regular polygons. Polygons are identified by name (Digon, Decagon, etc.) in function arguments and in results.

NumberOfVertices[p]number of vertices in polygon
NumberOfEdges[p]number of edges in polygon
NumberOfFaces[p]number of faces in polygon
Vertices[p]list of vertex coordinates for polygon
Area[p]area of polygon when edges have unit length
InscribedRadius[p]radius of the inscribed circle of polygon when edges have unit length
CircumscribedRadius[p]radius of the circumscribed circle of polygon when edges have unit length
Faces[p]list of faces from vertex numbers for polygon

Geometrical characteristics of polygons.

Digonpolygon with 2 edges
Trianglepolygon with 3 edges
Squarepolygon with 4 edges
Pentagonpolygon with 5 edges
Hexagonpolygon with 6 edges
Heptagonpolygon with 7 edges
Octagonpolygon with 8 edges
Nonagonpolygon with 9 edges
Decagonpolygon with 10 edges
Undecagonpolygon with 11 edges
Dodecagonpolygon with 12 edges

Names of polygons.

The functions Area, InscribedRadius, and CircumscribedRadius give information for a polygon with edges of length 1. The list of coordinates returned by Vertices is conventional for the specified polygon and does not necessarily correspond to a polygon with unit edge length.

This loads the package.
An octagon has 8 edges.
This is the area of an octagon when the length of each edge is 1.
These points represent the coordinates of the vertices of an octagon.
Here is a plot of the vertices of an octagon.