There are a total of 60 Poe's scattered around Hyrule a majority of which can be seen in the overworld at night time with there lantern being the only visible indicator of there prescence without Wolf Link's heightened sensing capability. When the player acquires 20 of these souls they can return them to Jovani for a bottle of Great Fairy Tears however the curse on Jovani will not be fully removed as he states and proceeds to request the player to return with the remaining souls to fully restore his body.
Interestingly the game does not actually check if the soul has been collected to update the counter in the collection screen instead it only checks for if the Poe has been defeated as there is a flag attached to this occurence. This can lead to the ability to gain more than one soul from the same Poe skipping the need to search for each individual one. In the randomizer this behavior is corrected by changing the soul counter to check for each soul collected and updating it that way and does not kill the associated Poe if the player finds the soul in a different location.