Not per plate. A private in-home dinner is one flat price for the evening — $300–$400 — and that covers up to four guests. Bigger tables add $55 a guest, because a larger group genuinely means more shopping, more prep, and more courses going out at once.
So a party of six is about $510, ten is about $730, and twenty is about $1,280. Notice what happens per head: $85 each at six, $73 at ten, $64 at twenty. The base spreads out, so the more people you seat, the less each one costs. Groceries are billed separately at cost with receipts and no markup.
You get one firm number in writing before anything is booked. No per-head service charge on top, no gratuity expected, no surprises on the night.