The number is not arbitrary. In the novel:
While the game does not ship with one hundred and eight unique named companions out of the box, the term has become a community shorthand for a specific style of play: the "Hero Army." Whether through the rigors of vanilla recruitment or the expansive possibilities of modding, building an army composed entirely of named, unkillable companions is one of the most exhilarating ways to experience the game. mount and blade warband 108 heroes
You want to kill 500 soldiers with a lightning spell, recruit a drunken tiger-slayer, and conquer China with an army of pandas. The number is not arbitrary