For many years, there were two sets per year. One based on the previous year's Hidden Mickey pins, and one based on some random theme. They also used to be $1.95, and they started raising the price. I think they are $3.95 now. But I think a lot of people stopped getting them as frequently and so they take longer to sell out. It's been a couple years now that they've only had 1 series, and no Hidden Mickey themed series. DLR and WDW sometimes share the "random theme" set, and sometimes they complement each other (a couple years ago, they had a ticket series, and the DLR ones said 1955, the WDW said 1971. WDW had Figment, DLR had Jack Skellington). They've had the pocket watches for almost a year now.
Hidden Mickey series: Last sets (one for each resort) came out in 2013. They were previously released also 2010 - 2012.