The short answer is: your guests will not go home hungry. How do I know this? Because you are a concerned host, you're already fretting about not making enough, and the rookie mistake is to make way too much food. That being said, here are a few tricks you can use to hit pretty close to the mark of having the perfect amount of food and minimal leftovers.
1. Don't triple that recipe!
Far too often people make a multi-course meal and quadruple each dish so that they have the precise number of servings for each person. The thing is, when a cookbook tells you that a risotto dish feeds 6, they mean it will feed 6 if that's the only thing they're eating. Your average diner will not be able to eat a full serving of risotto AND a full serving of soup AND a full serving of fish AND a full salad AND a full dessert. Instead, think of a dinner party as a sit-down buffet where people will want lots of little tastes.
Of course, there are notable exceptions to this rule; food that comes in individual portions, e.g. anything served in its own shell, in a ramikin, on a piece of bread, on a chicken bone, etc., will have to be made with the specific number of people in mind. It's important to remember that self-portioning foods like these pose challenges that ladled foods don't - and you will be reminded of this the next time someone shows up forgetting to tell you they're bringing four of their cousins from out of town, or the next time five of your guests are allergic to the main ingredient in the individual souffles that you just pulled out of the oven. Proceed with caution - or limit dishes like this to one per meal.
2. Do plate the food yourself!
Plating my guests' food might make me a control freak, but there's a method to the madness. If you're concerned about running out of food, or conversely if you're worried about people gorging themselves and then running out of room before they get to the dessert you spent six hours making, doling out attractive portions yourself will help solve either problem. It's a fact: human beings have really bizarro ideas about portion size. Think of the last time you ate at a fancy restaurant - odds are you looked down at your modest portion and worried that you'd be hungry later, and instead, after savoring every bite you left the table feeling perfectly sated instead of doubled over in abdominal pain. When planning your dinner party, think a little more high-end bistro, a little less Old Country Buffet.
3. Make at least one high-bulk item.
Soup is great because you can make it in bulk, it's relatively inexpensive, and it fills people up like crazy. Pastas and rice dishes are similar - and in a real emergency you can head back to the kitchen and double the pasta recipe. It's a good idea to have at least one major dish you expect to have leftovers of. That way your guests who brought their serious appetites can come back for seconds.
4. Stock up on emergency snacks.
In my family, holiday meals take about eight solid hours - after a feast we sit around the table long enough to get peckish, and that's when we bring the appetizer trays back out. Think of a well-stocked pantry as your insurance policy.
One of our readers, Jason Cristofaro, an excellent party host in his own right, suggests keeping the following items on hand:
Non-perishable (these things last forever!):
Crackers - a few different types
Homemade preserved veggies - pickles, eggplant, sun-dried tomatoes - it helps to have an Italian father!
Fancy jellies - We found this place in Nova Scotia, which is unbelievable. A bit pricey, but they will last you a while and are worth it. The horseradish jelly is awesome.
Boxes of Girl Scout cookies kept in deep freeze (people go apeshit over these for some reason; whenever we put out some with desserts we made from scratch, these will invariably be eaten first!)
Fig spread - goes terrific with cheese and crackers.
Nuts - you can roast mixed nuts yourself with seasoning if you want to get fancy. Marcona almonds are especially awesome, especially if your guests have never tried them.
Cheese - you can't go wrong keeping a nice hard cheddar and a brie (and they are virtually non-perishable if kept in the fridge)
Preserved meats - a dry sausage is great here. Again, having an Italian relative is awesome here if you want homemade.