Is there a chart or a guide as to how much cake is needed for a specific amount of people? For example if you need to do a cake for 50 people what sizesizes could acheive that?

when i bought my pans set there was a chart I hope it helps you, oh and i believe the size for each serving is 2" by 2".

6" 14 serv.
8" 12 serv
10" 39 Serv.
12" 56 serv.
14" 63 Serv.

you do the math and see what could work for you.

I use wedding size servings . I have a template in my photos that lists all the servings based on Wiltons wedding serving chart.

Here is the serving template.
There are a few others in my photos that may be of some help to you.

Wilton has charts which give layer cake servings yields by pan sizes and serving sizes:
(Party servings are 1-1/2x2x4 and Wedding servings are 1x2x4. Although some member use wedding servings for all their cakes because it does simplify explaining pricing.)

http://www.wilton.com/cakes/making-cakes/

HTH