Hollow Knight has a variety of ability-enhancing charms located all over Hollownest. Considering how difficult some of the areas can be to explore, having as many charms notched as possible is essential for surviving, dealing powerful attacks and maximizing Geo acquisition. Players start the game with three free charm notches in their inventory, which seems generous, but considering there are over 40 charms available, the Knight definitely needs more notches.
There are a total of 11 charm notches available in this indie Metroidvania, which means eight are out there for purchase, discovery or reward. Finding them as quickly as possible means getting to work opening new areas to explore and unlocking new abilities that take the Knight to the next level.
Salubra's Shop
One of the easiest places to find charms and notches is in Salubra's shop near the bottom of the Forgotten Crossroads above the abandoned village. Salubra is a charm connoisseur, and she respects that the Knight shares her interest enough that she offers notches and charms for purchase. The first notch upgrade can be purchased from her for 120 Geo after the Knight has collected at least five charms.
As the Knight progresses and his collection of charms grows, he can return to Salubra when he has acquired 10 charms and purchase another upgrade for 500 Geo. When the player has 18 charms, they can go back to her shop and purchase a third notch for 900 Geo. The fourth notch from Salubra can be acquired for 1800 Geo after 25 charms have been found or purchased.
Fog Canyon
This charm notch isn't easy to get to, as it's located at the end of a maze-like path in Fog Canyon beneath the Stone Sanctuary. The Knight needs Isma's Tear to swim in acid without taking damage or the Monarch Wings, which unlocks a double jump. To make things more difficult, there are plenty of explosive Oomas along the path, and the area itself is blocked by explosives that need to be detonated use Shade Soul before even attempting to move forward.
Fungal Wastes
The Knight can acquire a charm notch in a room located to the left of the uppermost bench in the Fungal Wastes. All he has to do is battle the two Shrumal Ogres guarding it and defeat them. Once they've been defeated, simply pick up the charm notch. The battle is an easy one, but as with all battles, proceed with caution. Fortunately, there's a bench nearby for a respawn, so make sure to take a seat there before heading into battle.
Colosseum of Fools
The Colosseum of Fools opens up three challenges. As a reward for completing the easiest of the three, The Trial of the Warrior, the Knight receives a charm notch. It costs 100 Geo to unlock the trial, and there are 12 waves of slow moving enemies to fight, each one with their own special strategy. This trial can be completed multiple times, though the charm notch is only rewarded once.
Grimm Battle
The Grimm Troupe DLC introduced the Nightmare King and Grimm -- and an opportunity to gain an eleventh charm notch. After lighting the Nightmare Lantern, return to Dirtmouth where the Grimm Troupe set up tents. Grimm gives the Knight a task to collect Nightmare Flames with his Grimmchild. After the second set of three flames is collected, he will challenge the Knight to battle to test his strength for their confrontation with the Nightmare King. Upon completing this challenge, he rewards the Knight with a charm notch upgrade.