About 186,000 advance tickets were sold for the 1969 Woodstock festival. Organizers expected a much smaller gathering, so the advance sales were already a warning that attendance could overwhelm the planned facilities.
Advance tickets cost $18 for the three-day event, while tickets at the gate were priced at $24. Sales were handled through selected record stores in the greater New York area and by mail through a post-office box in Manhattan. The organizers had initially anticipated roughly 50,000 people, but hundreds of thousands ultimately converged on the site.
The festival became effectively free when the promoters could not finish the fences and ticket booths before opening day. That is why 186,000 is not the attendance figure: it counts tickets sold in advance, while estimates of the crowd at Max Yasgur's farm are far higher. The resulting traffic, shortages and mud became defining parts of Woodstock's history.