How can I store the new SVN revision number in my source code after I commit? (TortoiseSVN)

Subversion does support keyword expansion, but you may want to read here before choosing to implementing it.

There are a number of reasons why it's generally a good idea to avoid modifying your committed files with content that is already in SVN. The second link above provides more detail on why this can be a bad idea. But basically, diffs and patches can become problematic.

You also don't need to duplicate the information that Subversion captures because this can be easily obtained via the log command.

SVN Special keywords - specifically: "$Revision$"