They should make stones for 10 or a bit more monsters but just the more important ones, then if the theme gets bigger they might add more monsters but probably not stones, the stones could be only for the main characters like:
Vampire: red
Werewolf: orange
Headless horseman: yellow
Swamp monster: bright green
Monster, crazy cientist: dark green
Invisible man: dark blue, emblem might be a hat and glasses.
Ghost: light blue
Mummy: purple
Zombie: pink
Skeleton:black