Ignoring page numbers in backup slides

This can now be achieved with the following option on all "backup" slides:

\begin{frame}[noframenumbering]{My Title}
\end{frame}

Source

This will cause the final number (e.g. 25/25) to be displayed on such pages.

As always, a matter of taste.

This can be done with the appendixnumberbeamer package. Just add \usepackage{appendixnumberbeamer} to the preamble and use \appendix before the first backup slide.

I have defined two commands to do this:

\newcommand{\beginbackup}{
   \newcounter{framenumbervorappendix}
   \setcounter{framenumbervorappendix}{\value{framenumber}}
}
\newcommand{\backupend}{
   \addtocounter{framenumbervorappendix}{-\value{framenumber}}
   \addtocounter{framenumber}{\value{framenumbervorappendix}} 
}

You can then use \beginbackup and \backupend before and after your backup slide to adjust the number of slides.

For my beamer template I also like to add

\setbeamertemplate{footline}{
  \leavevmode%
  \hbox{%
  \begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
    \usebeamerfont{author in head/foot}\insertshortauthor~~(\insertshortinstitute)
  \end{beamercolorbox}%
  \begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
    \usebeamerfont{title in head/foot}\insertshorttitle
  \end{beamercolorbox}%
  \begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
    \usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
    \insertframenumber{} \hspace*{2ex} % hier hat's sich geändert
  \end{beamercolorbox}}%
  \vskip0pt%
}

in the definition of the \beginbackup command to hide to total page number in the backup slides, otherwise you'll get something like "24/18".

Using all this, your slides will be numbered like "x/Y" for all slides before the backup, where Y is the total number of slides before the first backup slide, and the backup slides will continue the numbering of the previous slides.

To manually fix the total frame count to a certain number, say 25, you could add the following command

\renewcommand{\inserttotalframenumber}{25}

right after the \begin{document} command.

You can also add the \appendix command right before the beginning of your backup slides, so that the corresponding sections/subsections do not appear in the table of contents/navigation structure.

It should be possible to tweak the renewcommand above so that it automatically uses the last frame number before the appendix, but I don't know how to do it.

Just insert

\renewcommand{\inserttotalframenumber}{\pageref{lastslide}}

after \begin{document}, and place the marker

\label{lastslide}

on your last slide.