2 people per quart sounds right to me, if this is the main entree. If you can find enough cold storage, it would be helpful to refrigerate the finished stew at least 24 hours in advance. The flavors get a chance to meld together and any fat will solidify on top, making it easier to scoop off. Bring to room temperature before reheating. Good luck!

First, think gallons instead of quarts.
Google a recipe site that advises on large quantities because the amounts of seasons differ greatly when you increase from serving for four or six to 60+ !
Consider doing six batches of a ten serving size recipe.
Easy on the salt for larger servings.
This a great time of year for stews with all the root veg available.
Good luck.

First, I am going to assume this is a fairly thick, filling stew. You need to consider how many and what type of sides will be served as well. And who you are feeding (a bunch of little kinds mixed in or all adults or a bunch of teenagers that can all eat half their weight at every meal). I personally would plan on 2 cups per person assuming this is being served as the main course. So 2 people per quart for a total of 30-35 quarts. Plus some extra because I hate to run out.
This website looks pretty good for figuring out how much of each item to make. I can't say from experience how accurate it is though. http://www.dummies.com/how-to/content/cooking-for-crowds-for-dummies-cheat-sheet.html