How many lbs. of chicken and cans of broth will I need for 20 people?
It would be simple to estimate this way: how much chicken would you expect as a single serving? how much broth? Say you'd eat about 6oz, then 20 people would be expected to eat about 8lbs. Allow a little extra for big appetites - 2lbs extra, so 10lbs.
You'll need a lot of broth to cook the noodles in, so allow a can to 2 people. 10 cans. Easy to remember! Or you can cut down on the broth by pre-cooking the noodles in water with a chicken stock cube added. This can also be added to the broth.
One 3 pound chicken will feed 4 people. I'm not sure if you are cooking the noodles in the broth, so I can't answer the second part of the questions.
Figure 1/2# chicken per person. 1/4# uncooked pasta per person.
If you boil your own chickens, use your won stock so that you don't have to purchase broth. It will have no added salt, so will be healthier.