Download IsLog, a freeware application from Neocles, which returns the number of active user sessions on one or more WinFrame and/or MetaFrame servers.

The usage syntax is:

ISLOG.exe \[/V\] Username CitrixServerName \[CitrixServerName \[...\]\]

The number of sessions is returned in %errorlevel%.

Example:

IsLog.exe %UserName% CitrixServer1 CitrixServer2
if %errorlevel% GTR 1 ........

You can use a logoff program, such as LOGOFF.EXE, to force the user off, if they exceed your limit.