What mission chain gives two additional NPC slots other than the Stryll mission chain? I know it is possible to have 6 total NPC slots but am unsure which mission chain gives this as a reward. is it the lazuli mission chain? The decrypting the vault chain?

Thanks!

Zolazin and Lazuli. stryll just gives awsome cloak/scan modules.

Zolazin available at level 10 on the 4th mission.
Lazuli available at level 190. not sure which mission it is exactly.

Lazuli hasn't been fully documented in the wiki yet, so I don't see a mission reward for the 2 extra slots.

Yes, but nevertheless, that is where it comes from.