Three slices, then adjust
The working figure for a mixed group of adults is three slices each of a 14 inch large. It is not precise, because appetite is not, but it is the number that gets you closest most often.
Ten adults at about 3.2 slices each comes to 32 slices. A large cuts into eight. So:
32 ÷ 8 = 4 large pizzas
Adjust for who is actually coming
Teenagers eat more than adults — often four or five slices, sometimes more after sport. Children under about ten eat roughly half an adult portion. A group of eight adults and four kids is closer to eight adult-equivalents than twelve people.
Sides change it too. If there are wings, garlic bread or a salad on the table, drop the per-person count by about a quarter. People eat what is nearest, and pizza that arrives second gets eaten less.
Bigger is cheaper, and it is not close
This is the part that costs people money. Pizza area grows with the square of the diameter, while price grows far more slowly.
A 10 inch pizza has about 79 square inches. A 14 inch has about 154 — nearly double the food for a diameter only 40% larger. Two 10 inch pizzas give you 157 square inches, almost the same as one large, but they typically cost more and hand you twice as much crust perimeter for the same amount of topping.
Unless you genuinely need two different toppings, the larger size wins on almost every menu.
Ordering for a crowd
Spread the toppings rather than optimising them. For four pizzas, two pepperoni, one plain cheese and one vegetable covers most groups without anyone going hungry. Plain cheese is the one that always disappears, and it is the safe option for children and picky eaters alike.
If anyone in the group is vegetarian or avoids pork, make one whole pizza work for them rather than half of a shared one — shared pizzas get picked over before the person they were meant for reaches the table.
Round up by one, not by a third
When the arithmetic lands between two numbers, add a single pizza. Cold pizza is genuinely good the next day, and one spare costs a few dollars. Scaling the whole order up by a third because you are nervous is how a party ends with four untouched boxes.