I have a lady that is interested in an octogon shape cake. I haven't been able to find a website that will give me an idea of the servings for these cakes.

Should I just base it off of a round cake of the same size?

Not sure...

a 2 layer 9" HEXAGON is 20 servings and a 9" round is 24 servings so I bet you're in the 24-30 range for a 9" octagon. HTH

www.wilton.com/cakes/making-cakes/baking-party-cake-2-inch-pans.cfm

I think you mean hexagon...try this:

http://shinymetalobjects.net/cake/calculator/cake_calculator.cgi

Here is the cake chart from Collette Peters for round, hexagon or octagon shapes

6 in 10
8 in 20
10 in 30
12 in 50
14 in 70
16 in 100
18 in 125

i hope this chart can help you