I ran a bunch of events over a decade or so (in Australia), and found:

1. If the event was free, I would get roughly 66% attendance. Adding sponsored beer/pizza increased this, but only to about 75%.

2. If the event was paid, I got roughly 95% attendance, but a much lower audience, depending on price (and if the price was high enough, I'd get requests for refunds from people who couldn't attend).

If the venue space was limited, I'd overbook based on the above and it usually worked out OK.