How to increase attunement limit on Guardian Spirit?

[ЧВКашка]

my limit on Usura-Hicho is 14. And in the video I saw a player's limit at the same spirit is 20.

[Dweller Beyond the Threshold]

I think it just increases as you progress the game. I never did anything to increase the attunement limit of any of the guardian spirits, but when I started paying attention to what to equip, I noticed that they were already at their maximum limit.

So I think you just need to keep playing the game (like going on higher difficulties).