So far today I've already had a whack more people RSVP for the wedding. In short, this is why we build in a buffer whenever we estimate anything. For my wedding I ran with a 20% overage after I had thoroughly polled all my friends and family. At the time when I was defending the number it seemed ridiculous but based on today alone is probably not outside the realm of possibility.
Of course all of this said a buffer doesn't do you much good if you're rained out and thus no one shows up. Fortunately most food can be frozen. :)