It's maximum is 200. It's method is calculated like this according to Bulbapedia:
Base Power = 60 + (20x number of stat increases)
For example, if the target has used Bulk Up twice and Swords Dance once, there is a four-stage increase for the Attack stat and a two-stage increase in the Defense stat, making a total of six, which causes Punishment to have a base power of 180.
http://bulbapedia.bulbagarden.net/wiki/Punishment_(move)
