108 is also an important number in Buddhism as it represents the number of temptations/worldly desires you have to overcome in order to reach "nirvana". So it does have some religious context to the importance of the number "108". Buddhism is one of the major religions in Japan alongside Shintoism.I guessed 108 because it’s a very significant number related to ghosts and spirits in Japan. It’s Spiritomb’s entire schtick. It makes sense