It is useful to indicate the total number of slides in the header or footer of a presentation. This slide count can be added to a presentation created using LibreOffice Impress.
Go to View > Master > Slide master. This displays the master slide, which is used as the template for all slides in your presentation.
Click in the Slide Number Area in the bottom-right corner of the slide and choose Insert > Fields > Page count.
In the Master View dialog hovering over your slide, click Close Master View. You should see the slide count added to all your slides.
You can also add the current slide number in a similar way by choosing Insert > Fields > Page number.
Related: How to add slide number in Impress
Tried with: LibreOffice 126.96.36.199 and Ubuntu 12.04 LTS