Each key should open a new dungeon.
You also get a 'second chance' if you missed a key, as the higher level keys open the previous dungeons too. So if you found the Level 6 key in act 2 I think that it should open them all.
However, if you're in Act 1 you can find TWO keys for the same level 1 dungeon for some reason. So it's possible that you found the same key twice, if you see what I mean.
You should be able to check somewhere. I think you get a tiny number and/or pop-up on the square that you click to get to the battlefields from the main game. That should say what level keys you have. If it says 2 then 2 dungeons should open.
For some reason (on my patch version anyway) the numbering doesn't seem consistent if you are actually in the BF. In other words, in the main game it reports level 2 (or whatever) yet still says level 1 if you check when in the BF. Not sure if this is a bug, or if it refers to the act you're going back to or what.
Good luck. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />