Count confirmed diners before shopping
Write the three numbers the cook needs: “four confirmed, two possible, six seats.” Check portions and serving dishes against six, but decide which ingredients you want to buy in quantities for four. The shopping plan and the seating plan need not use the same number.
Make the extra meal workable
Set a time for the final count that fits the cook. If the invitation is still open after shopping, tell late guests what you can offer. If no extra guests come, leave the spare packs unopened and store them according to their labels.
In the taco example, buy enough tortillas and beans for all six or keep unopened supplies on hand. Keep the four-person ingredients separate on the list. That way “possible” does not turn into “another two full dinners already purchased,” and the extras still fit an ordinary lunch later.
Give late guests a clear answer
If two more people confirm after shopping, tell them what dinner can stretch to and when you plan to eat. If they decline, remove the spare chairs and put the extra food on a named meal plan. Do not count possible guests as confirmed when a table or paid reservation requires a final number.