As a shaman, you start off with two available ice cubes. Thus, with the skill fully upgraded, you will have a total of 7 ice cubes available for use.
These cubes are mainly used for trolling, to kill off the trapped mice or mice who refuse to finish the level so the sham can complete the level.
Ice cubes do not work on survivor maps. Ice cubes only work if a mice has entered the hole with a cheese.