So, no, there are not only 20 dragons - No one, not even Todd Howard knows how many dragons there are, so it's very much undecided. (Though, as of now, there are infinite dragons, as they spawn like any other creature.)
Indeed, it seems that people automatically assumed that if there's 20 shouts in the game (It's actually said to be over 20, but no matter.) and you get dragons' souls from killing dragons which are then used to learn shouts, then there must therefore be 20 dragons in the game, however, that is not necessarily the case, as it's not yet decided how many dragons there will be.
What makes my iron sword better than the iron sword wielded by the guy next to me? It's been said that you aren't the only one to be able to use dragon shouts. You may be the only one they fear and they truly see as a legitimate threat, but that doesn't mean you're the only one who can kill them. After all, in that case in order for the dragons to destroy all of Skyrim they just need to attack all cities at once, you can't be EVERYWHERE at once you know.
That's just what I was thinking, in the trailer, we see the Dovahkiin's sword hurting the dragon just fine, in fact, that's what kills the dragon, the dragon shout used in the trailer doesn't actually seem to hurt it much, and before someone says "The dragon fight was scripted!" I say "So?" the sword still clearly hurts the dragon, and why would Bethesda show te dragon being hurt by a sword if it won't happen in-game? So if the Dovahkiin's sword works against dragon's, logically, everyone else's should too, unless your sword is the only sword that can hurt dragons for some reason, and since what makes the Dragonborn special is dragon shouts, not carrying a special sword, I see no reason why that should be the case.
Now, we've been told that you are the one the dragons fear, but that doesn't necessarily mean you're the only one who can kill a dragon, it just means your the only one who stands a chance of being able to actually stop them permanently, or at least for a long term, I'd imagine.
Rules of Drama. You're the Hero?, you're the center of the story, you're special. If just anyone could do it, you wouldn't be Super Awesome Special Guy, and.... that'd be boring?
Except that doesn't mean only the hero can ever be able to do things. Most stories usually have more than one character who does things, it might be the hero who does the most important things, but other people can do things too. I certainly don't expect to see someone else defeat Alduin for me, but that doesn't necessarily mean no one else can hurt dragons but me, after all, I wasn't the only one who could hurt Daedra in Oblivion, now, generally no one else would kill Sixth House monsters in Morrowind, but that's because the surrounding NPCs would just stand around and watch while an ash zombie tried to kill me in my sleep, if you summoned monsters or otherwise gained companions, they'd still be able to help you kill enemies.