I assume you mean the sum of interior angles of a convex 8 sided polygon.
The formula for the sum of interior angles of a n-sided convex polygon will be (n - 2) x 180 degrees.
Hence, for a 8 sided polygon, the sum of the interior angles will be
= (8 - 2) x 180 degrees
= 6 x 180 degrees
= 1080 degrees
If you are asking for the sum of exterior angles of a 8 sided convex polygon, that will be 360 degrees. In fact, for any n-sided polygon, where n is 3 or larger, the sum of the exterior angles is consistently 360 degrees.
If you are asking for the sum of the angles of a concave 8-sided polygon, I am afraid I cannot help you, as the sum will vary greatly depending on the shape of the concave polygon.