This can be a tough one, but it seems to be the reality more often than not. Your venue and/or caterer will likely require a minimum count no later than 24-36 hours prior to your event. You will be required to pay the per person price for this number of guests even if fewer show up for your event, because your caterer has prepared food for this number. You may be able to add to your guaranteed number a day or so prior to the event, if you have a massive registration very late, but this is only true up to a point. Don’t plan to notify your venue when you arrive at 8am that you will have an extra 5 or 6 guests for lunch at 11:30. It MAY not be possible.
But when “guestimating” your attendance, keep in mind that if people are pre-paying for the event, they will likely attend. However, if they are not required to pay anything to register, it is quite likely that 15% or more will not show up.
When planning the menu for an event where the guest count is tentative, always go with the buffet. Many of our planners would like to go with the boxed lunch option, because it is less expensive; however, boxed lunches are finite. You cannot stretch a boxed lunch to feed more than one person without some really awkward conversations. However, when the same food in the boxed lunch is placed on platters and in bowls, it is quite rare to run out of food. Generally, guests will subconsciously take smaller portions if a quick check of the crowd indicates there may be a shortage. Somehow…there’s always enough and usually leftovers.