Here's the problem. Your caterer wants a final headcount about three weeks out, and most couples count the adults who said yes. Then the invoice shows up with the kids on it, and the photographer, and your DJ.
Your wedding website is already counting the replies. Zola, The Knot, Joy and Minted all track RSVPs.
Five boxes. That's the number you hand over.
*Note- Most vendor contracts say you feed them. Check yours, then count them below.