From the ZDayZ FAQ:
FAQ | ZDayZ 2012
In 2011, ZDayZ attracted 490+ registered attendees and 328+ Cars
In 2010, ZDayZ attracted 460+ registered attendees and 300+ Cars
In 2009, ZDayZ attracted 430+ registered attendees and 300+ Cars
In 2008, ZDayZ attracted 300+ registered attendees and 230+ Cars
In 2007, ZDayZ attracted 350+ registered attendees and 250+ Cars
In 2006, ZDayZ attracted 400+ registered attendees and 280+ Cars
In 2005, ZDayZ attracted 200+ registered attendees and 150+ Cars
In 2004, ZDayZ attracted 100+ registered attendees and 75+ Cars