The numbering system is pretty simple. The first number is the season. It is currently the third season for the Adventurers League, which is the Rage of Demons storyline. The second number is the specific adventure, so if is listed as DDEX 3-4, it would refer to adventure #4 from the third season. If it is DDEX 2-9, it would be the ninth adventure from last season.
As for characters, there are two different levels for the adventures. There will be pregens available for the games designed for characters of 1st-4th level (or 1st-2nd for 3-1), or you can bring your own Adventurers League legal character. There are specific character creation rules here: http://dnd.wizards.com/articles/events/adventurers-league-resources
. You do not need to have a character that had played in any of the adventures at this level. If you are interested in playing one of the games for levels 5-10, then you will need to have a character that has leveled up through playing previous adventures. There are no pregens for the higher level games.
Hopefully this has answered your questions. If you have more, feel free to ask, somebody should be able to help if I was unable to do so.