According to this discussion in Steam, a second pilot is useful to have, in case the first is killed, but in my experience, as the fighter is controlled by remote*, my pilot isn't killed when the fighter is destroyed. Maybe that used to be the case, and they changed it?
Unless they also made it possible to have two NPC pilots operating at the same time, and I've heard nothing that suggests that is so, then one pilot would be fine (and one bay, as you get six fighters per bay, which seems sufficient until you return to base).
steamcommunity.com
*it's possible that the pilot isn't even on my ship, but back on a station far removed from the conflict, like a real drone operator?
Unless they also made it possible to have two NPC pilots operating at the same time, and I've heard nothing that suggests that is so, then one pilot would be fine (and one bay, as you get six fighters per bay, which seems sufficient until you return to base).

Any reason to hire more than one NPC fighter pilot? :: Elite Dangerous General Discussions
You can hire up to three pilots for your fighter bay, but even if you've got one that's more than one fighter, you can only take one NPC with you. So, is there any reason to actually hire more than one fighter pilot? Are there circumstances where you could lose one and would need a replacement?
*it's possible that the pilot isn't even on my ship, but back on a station far removed from the conflict, like a real drone operator?