There is a certain level of anticipation and formality in declaring the number of strokes before punishment and it has a certain properness and ritual to it. Declaring your punishment lets you know how much you will be getting and no matter how hard you try, your mistress will not be letting you off easy without delivering the punishment he/she saw fit for your transgressions. It builds up anticipation and helps you mentally brace the punishment and get thru with it.
Most times I am punished, I am told beforehand what I will be getting and when I will be getting it and it makes me squirm in anticipation and nervousness with butterflies in my stomach for the time to arrive and the punishment to proceed and be over with. My Mistress knows my limits and how far She can push me so she declares her punishments as she sees fit and when she is giving it, she would make sure it counts.
So to me, declaring the number of strokes is a part of the punishment ritual and of course your Mistress or Master can punish you as they see fit without declaring the strokes or till they think you have had enough but declaring it beforehand gears them up to deliver the punishment with intensity with the amount of strokes they declared.
Perhaps why a 'six of the best' was so feared in schools because you exactly know what is coming to you and you know the Headmaster or Headmistress won't be doing you any favours while delivering it
