Over the years I have organised a number of events from boat parties to networking events. One thing seems to be similar, the number of people who do not attend.
Many people who organise events have to commit to a minimum number and there may even be financial commitment. My tip would be to always estimate an attendance 40% lower than the confirmed attendance.
Its always better for your future marketing to be either squeezing people, in or turning them away than to have an empty room or a financial penalty