Do you think that an 8in round x 4in high, 2 layer yellow cake with all buttercream icing, will serve 20 ppl? ...With nice size pieces...I see Wilton says it serves 24, but I don't think it would give nice serving sizes. I'm a little new to getting paid to make and decorate cakes and wouldn't want to make a cake to small for a party.
an 8in is more than enough for 20 people.
I would advise reading through the Cake Business Forum for guidance on pricing and starting out.
Also be sure to check and be sure if you can even sell to start with. A lot of states do not allow food sales from home kitchens.
That size will serve 20 slices that are 1 1/2 by 2 by 4 or 24 slices that are 1 by 2 by 4. If they want larger size pieces they will need to get a larger cake. Next time you have an extra cake, cut a slice each of those 2 sizes and see how big they actually are. People think they sound small but they look pretty nice when you actually look at them.