With a portion of chicken and gravy being about 1 1/2 cups. You can serve about 48 people with each 18 quarts. for 300 people you will then need more than 6 1/4 18 qt roasters. So, 7 roasters will be what you need for a minimum.
Most 18 quart roasters suggest you stick with 16 pounds or less. It depends somewhat on the shape of the bird. But it cooks best if there is 1 inch or more between each wall and the lid, and the turkey.
2
Common sizes in slow cookers are the 3, 4 and 5 quart size. For a family of four, buy at least a 4 quart unit. Depending on your family's appetites, a 6 quart or even larger unit might be appropriate.
quarters
A quart is 32 ounces. I would serve 4 oz portions to 8 people.
A Rival Roaster is an electric roasting pan. The 18 quart version can hold an 18 lb turkey. It costs around $50 to purchase. The main benefit to owning one, is that the oven is free to cook pies or side dishes during Thanksgiving.
4 people
What is a "people quart"
I would guess a quart of soup feeds three. I think one can is considered a serving, and a can of soup is typically about 10 ounces. A quart is 32 ounces.
1/2 quart = 1 cup A gallon = 8 pounds SO 1/2 a gallon = 4 lbs / 1 quart = 2 lbs and 1/2 a quart is 1 pound. 1 gallon = 16 cups / 1/2 quart = 1 cup. MB About 3 cups as The formula is true if cooked chicken weighs as much as water. But it doesn't. It is a little lighter and chopped or shredded makes it even lighter. So the correct answer is about 1 1/2-2 cups. Also 1 Cup=1/4 quarts (not 1/2)
"Quart" means 4, so there are 4 people in a quartet.
Chicken broth that has the water taken out of it before it is canned so that after it is purchased the buyer can put the water back in, or dilute it, which essentially makes the product easier and cheaper to sell than the original full water content broth as well as making it more flavorful by condensing it,