I cook 8 pounds dry per 100 for side dishes.
Do 2 pans for 50.
Each roasting pan: 7 cups (about 3 pounds) regular long-grain white rice, 3 1/2 qts (14 cups) very hot tap water (our water heater was set at 140F) or broth, stick of butter , and 2 tablespoons salt if not using salty broth. Each pan yields 28 cups (total of 56 cups or 50 servings)

If you add veggies (could be your mushrooms)
1 large white onion
1 large green bell pepper
2 large carrots, peeled
1/2 to 1 cup light olive oil or butter

Dice the veggies into 1/8 inch cubes and saute in a little of the olive oil.
Place the veggies in the hotel pan along with the rice and any leftover oil. Stir all of this together well. Add the stock (hot.) Stir again. Here you would omit the butter.

Tightly cover each pan with aluminum foil. Bake at 350F for 1 hour with no peeking. Let the rice stand from 5 to 30 minutes without peeking. I found that the 30 minute wait was better. If you have to stack the pans one over the other, the remove the bottom pan first and move the top pan down to the bottom rack. Bake that pan an additional 5 minutes.

You don't have to worry about scorching the rice and it turns out nice and fluffy.

Here is the basic roaster approach:

Soak the rice if necessary. Certain rice types, like basmati, are meant to be soaked prior to cooking. Older rice will also benefit from soaking, softening up the grains and preventing breaking. Soak the rice for roughly 30 minutes, and drain thoroughly to prevent adding too much water in the cooking process.
4

Preheat the electric roaster to 425 degrees Fahrenheit. Add your rice to the roaster with water/broth in appropriate ratios according to your rice type or package directions. Follow the standard ratio of two parts water to one part rice if no directions are present. Add additional water if the rice is not completely covered with water.
5

Bring water/broth to a boil. Let rice boil, five minutes for white rice, 8 minutes for brown or other whole-grain rice. Uncover the roaster and stir the rice. Re-cover the electric roaster and turn down the heat to 350 degrees Fahrenheit. Check on the rice after 30 minutes. If not finished, cook five to ten minutes longer.