From GameWatcher : “Sekiro: Shadows Die Twice’s Carp Scales are one of the most coveted items in the game. Technically a currency, the Scales can be spent on various items ranging from new gear to new attacks, and everything in-between. Here, you’ll learn about Sekiro Carp Scale locations.”
Source: N4G PC Sekiro Shadows Die Twice Carp Scales – Find All Carp Scales Locations