Extra npc slots are unlocked by the missions

• Zolazin Mission Chain---4th mission in the chain---The Glaviz Vendetta---20,900 Energy total
• Lazuli Chain---5th mission in the chain ---Tracing the Shipments---96,500 Energy total
• Sha'din Chain---5th mission in the chain---Renderer Recall---395,000 Energy total

Information from the wiki

I currently have unlocked the zolazin ones, and haven't started the other ones, but I find when I npc now at rank 550 my npc list often gets completely clogged with elites and bosses making it impossible to continue npcing and autoranking. It's obvious that I need more npc slots. Any recommendations out there when these missions should be completed by?

I'm in almost the exact same boat as you. *high fives* I also only have the Zolazin ones, and also have my slots regularly fill up. I think the Lazuli ones shouldn't be hard to get hold of, though the Sha'din ones are much more "Ehhh....". Probably a few weeks at most for the Lazuli ones, so they're definitely worth getting at our rank. Not that I have...

I figure Tactical Remodulation is a better investment than the Lazuli or Sha'din slots...

This^^

I unlocked Lazuli slots by around about rank 260 can't remember when i unlocked the zolazin ones. I got to 6 NPC slots as soon as i could then i started on the sha'din chain but didn't get very far still on the first mission (21/60). I decided instead to go for the tactical remodulation device from the exotic chain and i love been able to refresh by NPC's every 8 hours i do it once in the morning then again after work if no good NPC's are there.

Rank 298 Konqul Excavator, about 2761 max energy

Edit: I do plan to get all 8 slots by rank 400 though just a goal I have set myself no real reason

Huh, I haven't unlocked any extra slots at ranks 177, max energy 2221... Don't have much trouble w/ slots even so...

If you dont have the 30 bosses medal, its the perfect occasion to do so. While working on it, also invest some energy in the Lazuli chain.