If your sources are in svn, then they actually have the same svn- number, after a commit. But the numbers in the $Revision$ or $Id$ tags of your working copies are not updated by svn commit. However, if you do an "svn update" after the commit, it should update the numbers. So, you shouldn't need to work out the maximum value.
The svn keyword Revision or Rev or LastChangedRevision (they are all the same) are defined as the last revision where the file changed (see subversion book chapter 3 section keyword substitution). It seems to me that subversion does not have a global keyword that would give you the last revision number. But they do provide svnversion to get that number automatically (see subversion book highlighted box named "Where's $GlobalRev$?"). So Robin's solution below seems to be the best way to get the global version number.
svnversion> svnversion.tex ; texexec document.tex
in the Makefile, and on the document front page somewhere
\doiffileelse{svnversion.tex}{ Subversion revision \ignorespaces \input svnversion\relax \removeunwantedspaces . \crlf }{}