I'm sitting here waiting to see how many are coming for dinner. We will have either 4 of us, Michael, Christina, Ethan, and me or there could be 11 of us. (Could have been 13, but my inlaws already declined.) It is a good thing I'm pretty laid back.
If I was serving a large casserole type dish, it isn't a big deal to invite extras, but I'm grilling boneless pork chops. I have 9 of them. If 11 show up, I will cut the two largest portions in half. Plus, with grilled pork chops, I almost always serve refried beans. (Another source of protein, in case someone arrives who doesn't eat pork.)
Along with that, I have a large salad, baked potatoes, and a vegetable side dish. If a crowd shows up, I will pull out the bagels I have and make garlic toast or garlic bagel chips out of them. I like to keep appetizer items on hand too. Crackers & cheese, salami, veggies & dip, salmon dip or a variety of other things. I've even pulled out an apple and sliced it up real thin. One apple can go a long way. Peanut butter or caramel for dipping? You'd be amazed!
And if it ends up being just 4 of us, I will skip the appetizers and the garlic toast. The leftover baked potatoes will become twice baked for another meal. I will shred up the pork and make BBQ sandwiches, or quesadillas, or chili burros. I will find a way to use it to make another meal.
Often when I am not sure how many I will have for dinner, I put on a big pot of soup or a large casserole. But not always. Sometimes I have a meal planned at as the day progresses, the guest list grows.