I do this for a living and I can offer you this advice.

A regular 9x13 inch pan feeds 20-25. If you bake 6 of these you can put them all side by side and frost them as one cake.

You will need to use 1 1/2 boxes of cake mix for each cake to make them rise properly. Level the tops before turning out of pan to cool to make them all level.

YOu will need to put this on a huge platter or make your own cake board by cutting 4 pieces of cardboard from a large box. (Check your local stores for discards) Put them all on top of each other tape them together and cover with foil.

I can also give you directions on how to stack round cakes for a beautiful visual presentation. just let me know and I will post them! Hope this helps and let me know!