As you might know, it's possible to add multiple passphrases to the same LUKS encrypted filesystem. This means each one of these will allow you to decrypt and to mount your filesystem.
Show LUKS Key Slots
This is how you can check how many passphrases or file-based keys are currently in use for managing access to your encrypted filesystem:
[email protected]:~ # cryptsetup luksDump /dev/rhel/06 LUKS header information for /dev/rhel/06 Version: 1 Cipher name: aes Cipher mode: xts-plain64 Hash spec: sha256 Payload offset: 4096 MK bits: 512 MK digest: 85 84 e1 49 0f c3 7f df 9b fd 62 a0 50 c9 2a 14 a8 86 a8 0e MK salt: 58 e4 20 ed ac 01 56 cf cb b7 77 1d c6 11 20 6f 4c f2 fd 3a c5 c3 d4 f0 64 41 84 4c f9 58 a2 2f MK iterations: 87614 UUID: 9144c0fe-7758-47ac-886b-330ae8bfa096 Key Slot 0: ENABLED Iterations: 1418912 Salt: 65 fd 6c 78 84 7d 1b ae d1 42 13 0b f3 4c f7 41 9b 5f d5 e3 6e c5 a2 ce 05 28 02 f1 9b 56 07 b9 Key material offset: 8 AF stripes: 4000 Key Slot 1: DISABLED Key Slot 2: DISABLED Key Slot 3: DISABLED Key Slot 4: DISABLED Key Slot 5: DISABLED Key Slot 6: DISABLED Key Slot 7: DISABLED
As you can see, only Key Slot 0 is ENABLED and reporting some setup details. This means I'm only using one passphrase for my encrypted filesytem.
If and when I add more passphrases or encryption keys, they will be occupying next available slot, starting with key slot 1.