If the meal has to sit out in a warm place for any length of time, it is essential to go for a starchy foods. Quantities are always difficult, but you can ensure a plentiful supply of potato, bread or rice for a meal. Generally people eat far less than you expect ( unless they are teenagers or students home from college, when they eat far more than anyone would dream possible).
People moving round with a glass of wine in their hands and a small plate will eat less than if they sit down to table.
When cooking larger quantities of any food, it will take far longer than you expect to get large pans up to cooking temperature. You should ensure that the food is also well mixed so that it is evenly heated.
Domestic microwaves are only suitable for small quantities of food.
If you are preparing a barbecue for a large number, meat can be precooked in a conventional oven, and then quickly seared over the flame before serving.