You can use all Sundered Charms at the same time and even stack them.

Can you combine Charms in Horadric Cube?

These items only need to be present in the character's inventory and identified for their magical attributes to take effect. They do not function if placed inside your character's Stash or inside a Horadric Cube (even if that Horadric Cube is located in your inventory).

Do Sunder charms stack Diablo 2?

Sunder Charms can only be dropped by Rare, Unique, Super Unique, or Champion terrorized monster types in Terror Zones. This includes Uber monsters. Carrying more than one of the same charm does not stack the resistance reduction effect on monsters but does stack the player's negative resistances.