Poison damage is reduced by resistance, and not defense. I do not know exactly who's ninjitsu on the offensive team is used.

Poison can have up to 4 stacks, 2 stacks just do twice the damage of a single stack. All poison stacks are counted as a single layer for debuff clears, but poison has the lowest priority for debuff clear.

Poison damage is capped at 1999 damage per stack, this value cannot be exceeded. Under normal cir*stances, you cannot get anywhere near this capped value without being far higher power than your opponent or fighting squishy ninjas in arena.

It's 1999 per stack, for a total of 7996.