Nahiri's Wrath adds the mana values of the discarded cards in whatever zone they were put into as a result of being discarded, even if they weren't put into a graveyard (due to madness, for example) or are no longer in that zone.

The damage is not divided. Nahiri's Wrath will deal that much damage to each target.

You can't target the same creature or planeswalker multiple times to deal more damage to it.

If you discard a card with {X} in its mana cost, X is considered to be 0.

If you cast Nahiri's Wrath without paying its mana cost, you'll still choose a value for X and discard X cards.

If you copy Nahiri's Wrath, it has the same number of targets and deals the same amount of damage as the original spell.