It depends on which type of chicken you use, for example, a whole chicken will yield in approximately 6 portions. Also, gravy is a matter of personal preference, some love to make their chicken swim in gravy, others just want a taste. As a general rule, you should be good if you assume a quarter-cup of gravy per person.
The easiest way to do this would be to test your favorite recipe in the pans you are using and multiply it - ie. if one recipe will yield 6 portions, then you will need to make 30 times the recipe.