For the determination of X, does the game look at the cumulative damage per source during the turn? or at each individual instance of that source dealing damage?

example.
Pestilence is activated 4 times and Pestilance has dealt 4 damage cumulatively to each player and perhaps some creatures. Would X be 4 or 1?

a 4/4 double striker hits unblocked, is X 4 or 8?

The ruling doesn't explicitly indicate if it means:
a) the greatest single instance of damage dealt by a single source to a single permanent or player during the turn
or
b) the greatest cumulative amount of damage dealt by a single source to a single permanent or player during the turn.

11/7/2014 Impact Resonance looks at the entire turn to see the greatest amount of damage dealt from a single source to a single permanent or player. For example, if the only damage dealt during the turn was Earthquake, which dealt 5 damage to each of four creatures and two players, then X is 5.

Edited Todd Bussey (Feb. 10, 2015 02:03:30 PM)

A) is correct. Impact Resonance looks for the greatest single instance of damage dealt by a single source to a single permanent or player during the turn.

Each separate instance of damage is considered separately, so if Pestilence is the only thing that has dealt damage this turn, X will only be 1 regardless of how many times it's been activated, because each individual instance of damage was only 1. In the case of the 4/4 double striker, X would be 4.