What is the rule of thumb on headcount?

Just found an answer to my own question - The general rule of thumb is if you're having over 200 guests, then you can estimate that about 25-28% off your guests will be unable to attend. If you are having less 200 guests, then the percentage usually decreases to about 15-20% or less. Other factors include how many guests you invite that live out-of-town and the travel distance required to attend the wedding.