To add a little more flesh to the bones, it is important to realise that not all cabin crew are equal. The one per 50 seats rule refers to SEP (Safety and evacuation procedures) trained cabin crew. Often additional but untrained cabin crew, in SEP terms, are carried to do the hard work in the cabin. You'll not be surprised to hear they are paid less than their trained colleagues and also because they are not part of the "crew" they have no protection from any work/rest regulations. Not that any airline would ever take advantage of this.
PM