Casita here,
Originally there's only one set of numbers 7/10 representing the amount of workers the enemy ship has. So we didn't knew how many defenders there were in battle.
In the beta there's 7/10 + 8, with the "8" being the fixed amount of defenders not killable during ship battle.
From my ship battle experience it seems reducing the first set of numbers lowers the amount of enemies during boarding battle.
Keep in mind that starting a battle using a grapeshot to reduce the workers 7/10 to 2/10 in order to reduce total defenders is a bad idea.
Mainly because in few turns the workers rest out and jump back the numbers to 7/10.
Grapeshot them 100-200 m and then RAM THEM. That works :D