You should order for only 90% of your guests. Some people will not eat the cake. So again, only order for 90% - in your case that means 12 people or one cake box mix - or one 9 inch round/square cake.
for an 14 in round cake pan, you only need 1 box
Generally it depends on how deep (how many layers) your cake is. Standard serving size of a piece of cake is 1" x 2". Also depending on how large you want to cut your slices (sensible vs generous), a 7" round cake can serve anywhere from 6-10 people.
A ten inch cake feeds about 12 people if you cut it in wedge like triangles, like a pizza. However if you go to wilton.com and look at the cutting guide, they say it feeds 38, realisticly, more like 30. That is if it is at least 2 layers. and those are uniform peices so everyone gets the same amout of cake. I like this way. it makes the cake go farther and no one eats half a piece and throws away the rest. and if people want more they can always go get another piece.
An 8 inch square cake can reasonably be divided into 4 pieces, each of which will be 4 inches square.
It depends on how large the cake is.
About 2 boxed cake mixes.
it should take about 2-3 cups if its 6inx6in i would make 3 just in case
that depends on how many slices you want.
about 1 and a half
A 9x13" cake will feed about 45 people (assuming that the cake has two layers, each layer is about 2" in height, and the slices are 1.5x2" in size).
4 boxes for a single pan