2 dimensional (plane) figures can have three or more straight edges as sides. 3 dimensional (solid) figures can have four or more plane sides (faces). They can have fewer sides if the sides are not straight.

An octagon has 8 sides. There is a general formula for all plane geometric figures. The sum of the interior angles is 180(n-2) where n is the number of sides. So: 180(8-2) = 180x6 = 1080