The Thu'um Master achievement only says:
Learn 20 shouts
I want to know if I need to learn all three words of power for each of the 20 shouts or if the first one is enough? Until now I have learned several words from 19 shouts, but I am far from completing them. If I need all three words of power for each shout I might consider that it is not worth the effort. If I only need to learn one more word of power from an unknown shout, I will definitely go for it.