When it comes to planning a dinner party, what to serve and who to invite are not the only considerations. You also need to know how much to cook.
I'm a terrible person to advise on this. I recently prepared an afternoon tea at home for about 60 people. I made two Victoria sponge cakes, three bakewell tarts, two lemon tarts, two chocolate cakes, two sour cream cakes, three summer puddings, 36 scones, a couple dozen eclairs, 30 kouign amann (a flaky pastry from Brittany), 25 financiers, a pot of duck rillettes, three quiches and about 60 gougeres (cheese puffs). I also ordered two kilos of assorted raw-milk cheese, in case my guests were very hungry.
The problem is that only 35 people were invited, and only 25 actually showed up. I had overcatered - again. But even as guests were arriving and starting to eat, I was panicking, worried that I hadn't made enough.
I tend to think everyone will eat like I do when faced with a broad selection of food, which is to taste a bit of everything, find out which I like best and then go for seconds (and sometimes thirds). Anticipating that, I cut the cakes and pies into small pieces, to serve 10 to 12, but not everybody tried everything. I could easily have fed triple the number of guests. At the end of the afternoon, I was sending people home with goodie bags of food - a few took home whole cakes and tarts.
Some hosts underestimate the amount of food to serve; others, like me, overestimate. While the former is worse (it's not very hospitable if your guests go home hungry), the latter can be bad as well. Yes, it's nice to have leftovers for a meal or two, but sometimes when I cook, there's so much left at the end of the meal there's no way my husband and I could eat it before it goes bad.
It's easy enough to anticipate portion size when serving something simple, such as steak or chops - just count on one per person; while a chicken can usually feed three to four when served with side dishes. Cooking a meal of shared dishes - such as Chinese, Thai or other Asian cuisines - can be trickier; the standard way of ordering at a restaurant, about one dish per person, doesn't always work at home. If you're cooking for eight, it's extremely stressful for one home cook to try to make eight dishes and send them all out piping hot. It's better to make fewer dishes and serve larger portions of each.
For barbecues and other events where there's a wide range of meat, seafood, salads, starch and condiments, count the number of guests and the number of main dishes you plan to serve. If you're barbecuing four main dishes (such as hamburger, sausage, chicken and fish), rather than counting on one piece per person, you'll probably be safe if you plan on each guest eating only two of the four, or three if they're particularly large eaters; it's rare that someone will have one of each.
If you have events like this frequently, keep notes on how many guests you had, what you served and the amount, and how much was left over. That's what I did for the tea party. Next time, I'll make the same amount of food but double the number of guests; or more likely, I'll halve (at least) the amount of food I cook.