Strionic Resonator's ability only copies triggered abilities. Each instance of drawing a card triggers Nekusar's ability once, so the triggers are all for 1 damage each. You can get 8 damage total with Resonator.
Note also that commander damage only counts combat damage dealt by commanders. It does not include damage dealt by abilities.
February 16, 2016 3:42 p.m.
Nekusar, the Mindrazer's ability triggers for each instance of card draw. So after Windfall resolves, you will have X independent instances of the trigger (Where X is the greatest amount of cards that was drawn). You can copy one of them with Strionic Resonator making it X+1 instances.
Note that Windfall has to finish resolving before the triggered abilities go on the stack from Nekusar.