The answer is to ALWAYS RSVP! Whether you plan on attending an event or are unable it is a common courtesy to respond. The number of guests at any event is everything...how many chairs to set-up, how much food to prepare and how large of an area you will need to keep your guests comfortable. Once again, you are trying to produce a memorable experience - you don't want their experience to be that there was not enough food, no where to sit or "it was so crowded I left early." So this holiday season and throughout the rest of the year, be kind to your hosts and remember to always RSVP.