[From what I see here](http://www.uesp.net/wiki/Skyrim:Shout) it looks like 18 are learned without requiring dragon souls, though I'm extrapolating a bit since I haven't reached the end of the main quest yet, myself. 

Fuzzy math:

    Animal Allegiance     3 Souls    
    Aura Whisper          3 Souls    
    Become Ethereal       3 Souls
    Call Dragon           0 Souls* 
    *I'm making the assumption that this works a bit like Unrelenting Force
    Call of Valor         0 Souls
    Clear Skies           0 Souls
    Disarm                3 Souls
    Dismay                3 Souls
    Dragonrend            0 Souls
    Elemental Fury        3 Souls
    Fire Breath           0 Souls
    Frost Breath          3 Souls
    Ice Form              3 Souls
    Kyne's Peace          3 Souls
    Marked for Death      3 Souls
    Slow Time             3 Souls
    Storm Call            3 Souls
    Throw Voice           3 Souls
    Unrelenting Force     1 Soul
    Whirlwind Sprint      0 Souls

So...3+3+3+0+0+0+3+3+0+3+0+3+3+3+3+3+3+3+1+0 = **42**