From canon, I get the impression that no uses of dragon blood were known before Dumbledore's time; else there's not much scandal in Rita Skeeter's accusation that Dumbledore didn't discover all of them. So no uses known, Dumbledore publishes twelve uses, and then the rest of the world assumes that there is nothing further to learn.

Imagine MoR!Harry's exasperation on learning about this incredible complacence and lack of curiosity! He'll probably think of five more uses immediately when he hears about the first twelve (then worry afterwards about his Dark Side, since every one of them is a method of killing).

Now that's just strange - why would there be only 12 uses of dragon's blood?

When I read that I imagined that there something like 12 major magics with dragon's blood and that there was some underlying theory that made there be exactly 12 of them.