It says "upon defeating a dragon, skyrim's hero absorbs the soul of the fallen creature, which fuels his ability to LEARN a new shout". I interpret that as every dragon slain unlocks a "slot" to learn a shout.
Hmm. Not the 60 words, the 20
shouts. So that would make at least 20 dragons. I can work with that, was not far from what I theorized (one dragon-soul every five dragon-words).
Sorry for the confusion between words and shouts.