This is one of the most popular meals that I make for my cohousing community, and one I get frequent requests for. It's adapted from a Cook's Illustrated recipe, and all their meticulous testing really shows in the result. Thanks to an initial "enrichment" step, the broth has a deep complex flavor, even though it starts with purchased stock. If you have 40 cups of homemade stock lying around the house, by all means use it - it will be even better. But you really don't have to.
Tortilla Soup for 30
6 dozen corn tortillas
40 cups water 
1 jar Better Than Bouillon Chicken 
7.5 lbs bone-in chicken breasts and/or thighs, skin removed and trimmed of fat
5 large white onions, ends trimmed, quartered and peeled
1 head of garlic, cloves separated and peeled
1 bunch cilantro
5 sprigs oregano
10 medium tomatoes, cored and quartered
2 jalapeño chiles (optional) 
2-5 chipotle chiles in adobo (depending how hot you want it) 
3-4 tbsp vegetable oil (depending on the size of your pan)
1/2 tsp salt
4 limes, cut into wedges
6-8 avocados, diced fine
2 lbs of cheese (Cotija is traditional, but you can substitute shredded Monterey Jack or - better yet - put out some of each)
1 bunch cilantro, chopped
2 jalapeños, chopped fine
32 oz sour cream
rest of the can of chipotle en adobe, pureed into a sauce
Preheat oven to 425 degrees. Cut the tortillas into 1/3 to 1/2 inch strips. Working in batches, spread them on sheet pans, drizzle with vegetable oil and toss to coat, then bake for 7 minutes. Rotate pan and shake or stir strips to redistribute and cook for another 6-7 minutes or until golden brown and crisped. Sprinkle with salt to season.
In a large pot, combine 40 cups of water with the Better than Bouillon. Add chicken pieces, half of the onion quarters, half of the garlic, and the whole sprigs of cilantro and oregano to the pot. Bring to a boil then reduce to a simmer, cover and cook until chicken is just cooked through, about 20 minutes. Transfer the chicken to a platter to cool, and strain the broth into another large pot, discarding the remaining solids.
In a food processor, puree the tomatoes, remaining onions and garlic, jalapeño (if using), and chipotles until smooth. Depending on the size of your food processor, you will probably need to work in batches. Heat the oil in a large frying pan (or two) until shimmering, then add the tomato mixture and salt. Be careful - it will splatter! Cook, stirring frequently, until mixture darkens in color and starts to dry out - about 15-20 minutes (the smaller amount if you're using multiple pans, the larger amount if it's all in one).
Add tomato puree to stock and bring to a boil. Reduce to a simmer and cook for 15-20 minutes to blend flavors. Meanwhile, shred or pull the chicken into bite size pieces. Add it to the simmered stock and cook for 5 minutes to heat through.
Serve buffet style. At the start of the line, put the big bowl of tortilla strips. Follow that with the soup, then all of the garnishes.
 You could use canned or carton stock, but the Better Than Bouillon a) weights less, b) cost less and c) was the second place winner of the Cook's Illustrated canned stock taste test.
 When I make this, I use just 2 chipotle chiles for the whole batch, and no jalapeños, because I have to cater to a wide range of spice tolerance. Use the larger amounts below if you have a spice loving crowd!