How many proxy addresses can one user account have?

The answer varies according to the version of Windows you're using. In a pure Windows Server 2003 Active Directory (AD) forest in which all Global Catalog (GC) servers run Windows 2003, a user object can accommodate somewhere around 1200 proxy addresses. In an environment with Windows 2000 Server GC servers, the maximum is somewhere around 800. In practical use, of course, you probably never want to have anywhere near that many proxy addresses. No production systems that I know of use more than a handful.