How many hidden slides does my presentation count in PPT 2013?

You can count in the thumbnails on the left of the presentation screen as they are marked with a grey rectangle with the number crossed by a diagonal line. But that can be a pain, slide sorter view can show you more slides but even then it may not be practical if you have too many slides.

Fortunately you can see the number of hidden slides in the properties of the presentation. Just press File - Info, on your right you will see the details with amongst them the number of hidden slides.