Offering 4 of them at a Sculptor's Idol will increase maximum Vitality and Posture.
In total, there are 40 prayer beads that are attainable in the game. With 4 prayer beads, it is possible to create a Prayer Bead Necklace, which increases maximum vitality and posture. Awarded for defeating bosses and minibosses. Sometimes also found in chests:
After beating Snake Eyes Shirahagi, going into the cave behind her and looking up Wolf will see a grapple point. Upon proceeding, Wolf will have to crouch to get past 2 low hanging stones. Stopping between the two of them and jumping up the cliff, Wolf will be able to wall jump up to the top of a statue, where he will find the prayer bead.
From the Water Mill Sculptor's Idol, going through O'rin of the Water's arena, over the bridge, and up to the house, there is an opening to the right-side of the house's stairs Wolf can crouch under. Proceeding under the house, the Wolf will encounter shinobi floorboards. Going outside the house and grappling to the 2nd Floor, the item can be found on a shrine.
In the arena with Long-arm Centipede Giraffe, there is a hole in the ground. Drop down, crouch, keeping to the left Wolf will come across a chasm. Looking up, there will be multiple grapple point, and keeping to the right will lead Wolf to the Prayer Bead