I'm reminded of a time I had 17 people staying at my house, age grandparents to little kids. Meals for a lot of people are best if you keep it simple, and if you just put out all the fixin's, they can just make it to suit themselves. Day 1: Sandwiches - just put out meats, cheeses, bread, lettuce, tomatoes, etc. Add chips or a few sides. Day 2: Fajitas - I just grilled chicken, beef, onions, peppers. Put them out with tortilla wraps, shredded cheese, lettuce, salsa, and sour cream. Add tortilla chips or other sides. Day 3 - Big pot of spaghetti and meatballs and sauce (all separate of course); fixin's for green salad with a few dressing choices. Day 4: Eat out - everyone on their own, we just directed them to burger or pizza places near us. Day 5: Big roast turkey feast with stuffing, mashed potatoes, green beans, pies, like it was Thanksgiving (it was actually spring). It was easy because I'm used to cooking that exact food at least once a year. That was our actual meal plan for a week-long visit. It was a great time!
If you're talking more than 20 people, that's when I might call a caterer...or you can make it a potluck where people bring something.