One cannon is equal to your Cover Fire. When Livonah activates, she reduces 2x your Cover Fire from your Pirates. Who gets the reduction is based on who got affected by Scurvy.
For instance, if Livonah activates and her Skill affects 2 Pirates, both Pirates lose 1x your Cover Fire. If she affects 4 Pirates, those 4 Pirates will lose 0.5x your Cover Fire.
Cannon mechanics can be a bit tedious to understand, but I hope this explanation made things a little more clearer for you