At free corporate events the average no-show is between 20 and 40%. At paid events the no-show is much lower: 5 to 15%. The difference comes down to the financial barrier: those who have paid turn up.
Within the free category the no-show varies widely. An internal team day has almost no no-show. A non-committal client event can have a 40-50% no-show.
With an active turnout strategy (a strong invitation, reminders, personal confirmation) you quickly drop below 20%.