Wiltons chart says that a two layer 8" will serve 25 and a two layer 6" will serve 14.
Wiltons chart says that a two layer 8" will serve 25 and a two layer 6" will serve 14.
My Wilton chart says a 6" 2 layer will serve 12 and an 8" 2 layer will serve 24.
Each layer being 2" (4" tiers), for a total of 36 servings. 
I think I will make mine a 10" & 8" also. Thanks so much for the advice!
A 10" two layer and a 6" two layer would be closer to the number of servings that you need but it never hurts to have a little extra cake. 
Here's a helpful chart from Wilton on batter amount, cook times, and servings.
http://www.wilton.com/cakes/making-cakes/baking-party-cake-2-inch-pans.cfm
You'll see even from just this thread that serving guides never seem to quite match each other though, always off by a few peices, lol. Good luck with the cake!
One more question.....
Should I use a 12 or 14 inch base (for the round 10" & 8")?
I would use the 14 inch but either one would work. If you're using the cardbord cake circles then you need to tape 3 or 4 of them together and cover with freezer paper, Wilton fancifoil (not regular alu. foil), decorative paper /clear contact paper, fondant,ect. You can wrap a ribbon around the edge of your cakeboard as well. Lots of ways to make it pretty 
One cardboard circle isn't going to give your cake enough support and can cause your cake to split or break when moved.
You can buy cake drums but I can't find them locally. I usually just tape a few cake circles together and they're plenty strong enough for my 3 and 4 tiered cakes.
Good luck and be sure to post a picture.
I used 10 and 8 inches !
I know, I'm just saying based on what you are making.
I was giving two examples.
Kitagrl, I usually do the same. It looks prettier in my opinion.
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