Also, for 300 people, you are talking up to 25 gallons of soup. Consider using one of the top quality frozen soups, such as are used in many good mid-level restaurants.

Per 100, you need about 6 gallons of broth, 7 pounds of meat (which is 1 14 pound turkey, or 15 pounds whole chicken.) 1-2 gallons canned tomatoes with juice, 2 quarts onions and celery, and about a #10 can each of corn, green beans, carrots and peas, or equivalent frozen, 2 pounds noodles, parsley bay leaf etc to taste. This makes a rich soup and plenty of it for 100 people.
Also, turkeys make a better choice than chicken for this much soup.