There are 20 shouts in Skyrim, each with 3 levels of power. However, some of these shouts require dragon souls to unlock, and others are simply given to the player as gifts / quest rewards. (e.g., Clear Skies, Call Dragon, Call of Valor et al)
My question is:
What is the minimum number of dragons you must kill to be able to possess all 20 shouts at their full power?