There were reports last year of several parties selling out. The very first party sold out, as did the parties for Gay Days and UEA. Also, we went on 10/23, and there were signs in the Esplanade saying that the party was sold out. Halloween night has sold out for the past 2 years. My info is based on reports from last year on other boards where posters were desparately seeking tickets for sold out parties for a handful of dates and from personal experience of seeing the sold out signs in the esplanade on more than one night.
But even if a party does sell out, it doesn't usually happen until much closer to the actual party date.