Graduation parties are usually open houses, so guests drift in across an afternoon rather than arriving together. Plan for the full guest list with a buffer for the waves, and lean on food that holds well over hours: pulled meats, slider stations, big bowls of pasta or potato salad, and a sheet cake that serves a crowd from one pan. The graduation calculator sizes that spread for your count; a taco bar or walking-taco station is a popular self-serve centerpiece that keeps you out of the kitchen.
Because the room is never quite at full capacity but never empty, the trap is sizing for the crowd standing in the yard at any one moment instead of the total who will pass through. Build the list for everyone invited, set out self-serve food in refilled trays, and keep the buffer in place so a late rush of relatives does not catch you short.