http://wiki.guildwars2.com/wiki/Dungeon
8 dungeons in total, with 4 modes for each dungeon. Seeing as you will be side-kicked down to dungeons of lower levels they will remain useful even at 80. So essentially we will have about 32 dungeons at launch. Put into their that even in the dungeons there can be dynamic events and every run will feel a bit different.
I want to emphasize this because I know it is hard to understand, especially after having played WoW for so long. End game isn't gear grind. At least not for stats. For looks yes, but all 80 gear of a particular quality will be the same value and will be capped at that for everyone. So once you get your functional gear, then you go for appearance and get all your skills unlocked. Then it is about maxing crafting if you so wish and mastering your character.