The number moves and the shopping cannot. A confirmed count with a deadline written into the booking is what stops a caterer buying for forty and serving thirty-one, which is the whole margin of a private event.
Recipes have to be costed to know anything. Ingredient prices per portion, scaled to the guest count, are what turn a per-head price into a profit rather than a hope, particularly when a supplier price moves.
Dietary information must survive the whole chain. Collected at enquiry, confirmed a week before, on the kitchen sheet and on the plate: losing it at any point is the one failure that matters.
Staff and equipment are booked per event. A dinner for eight needs one person and a christening for forty needs four plus hired equipment, and both are commitments made weeks in advance.