Skill-based damage display is capped at 999,999.
Melee hit damage display is capped at 32,768. (THIS INCLUDES DOUBLE ATTACK, WHICH GETS CAPPED AT 32,768 TOTAL AKA 16,384 PER HIT).
This is why sinxs always hit 32k/16ks.
The actual damage is uncapped. You could theoretically hit up to 2,147,483,648 damage per hit, iirc.