This achievement is tied to Salubra, an NPC found in Forgotten Crossroads:
In order to be able to reach her hut you must have the Mantis Claw from Mantis Village.
She sells Charms and Charm Notches and once you have 40 charms in your inventory she will offer Salubra's Blessing for 800 geo. Buy this item from her and the achievement will unlock.
You can obtain 40 different charms:
As long as there is a charm in each of the 40 inventory slots, Salubra will offer her blessing for sale. It does not matter whether you have the Fragile or Unbreakable versions of Leg Eater's charms, if you have Grimmchild or Carefree Melody (dependent on how you advance the Grimm Troupe questline), or if you've turned your Kingsoul into Void Heart.
For locations and info on each charm, see the Charms page on the Hollow Knight wiki