I don't think so, you get dragonshouts from killing dragons, and there are 20 dragonshouts, ipso facto, 20 dragons.
Dragon shouts = New Sigil stones?
Also, just because there are only 20 dragon shouts does not mean there are only 20 dragons. Case and point? Again, sigil stones.
Also, Todd said that dragons would be making quite a lot of appearances in game, indicating that perhaps they don't stop spawning until the main story ends. So to only have 20 dragons would go against this concept.