The slide number can be displayed by using the Slide number option in Header and footer. But it is also useful to display the total number of slides below every slide. For example, by combining it with the current slide number: 3 of 10
How to add the total number of slides to the header or footer of every slide in an Impress presentation?
First, you should get yourself familiar with Master slides. Read Chapter 2 of Impress guide, if you haven’t already.
Then, when you do edit Master slide, put cursor in “Slide Number Area” (usually in lower right part) and choose Insert → Fields → Page Count. Provide additional text or formatting as needed.
Of course you can put page count field directly at your slide, but then you must ensure that it does appear on each slide, preferably at the same place. Master slides were invented to take away that burden from you.
Completely agree: very useful to firstly get started with the Master Slides in the Impress Guide, so that one can get the basics straight.