LibreOffice Impress is a good tool to create presentations. It is a good habit to have slide numbers in your slides. This helps the audience keep track of progress and also to refer to a particular slide.
To add slide numbers:
LibreOffice 4.2.x: Go to Insert > Page Number > Slide
LibreOffice 4.4.x: Go to Insert > Header and Footer > Slide
Check the Slide number option.
The above option adds a slide number on the first slide too. Typically, this is the title slide, where a slide number does not look appropriate. If you feel so, then check the Do not show on first slide option.
Remember to click Apply to All. If you click Apply instead, it will apply only to the current slide!
Tried with: LibreOffice 18.104.22.168, LibreOffice 22.214.171.124 and Ubuntu 14.04