I defeated 1 Pirate fleet and received 1 Agent.
Defeated a second one but did not get a second Agent.

Why?

I want my second Agent.

that's not what the quest says.. tbh i dunno what it was and i can't check it now cause it's completed but it was completely random for me..
i think it said hire another agent, maybe you need to hire another officer...

i suggest you keep playing untill you get it aswell randomly..

To complete 5th mission you have to defeat a pirate fleet that is worth at least 100 stat points. If you have beaten such a fleet, but the mission wasn't accomplished with that, you should post your report to "Bugs" forum.