I'm having a party this weekend for about 20 people - is risotto ever possible for that many? i would love to be able to make this recipe.

I'd use Merrill's instructions for cooking the farro risotto. Her recipe calls for 5 cups broth to 1 cup farro, while the spring risotto recipe uses 3 cups broth to 1 cup rice. http://www.food52.com/recipes/9113_farro_risotto
I'd do a trial run of the recipe, to make sure the farro cooks as expected, and get a handle on how much it makes so you can scale appropriately for the party.

You could do it for that many people if you have a large enough pot. I'd consider changing it to a farro "risotto" (unless there are gluten issues) so you could make it ahead to the point of adding the veggies, then finish it at mealtime. If you use rice it doesn't hold as well, and you should read through the cooking pointers in this thread: http://www.food52.com/hotline/11571-how-do-restaurants-make-a-risotto-that-they-can-serve-all-night-i-want-to-be-able-to-make-a-risotto