well i count this # on a site i made. But it takes advantage of the events on NT servers only,
in the Global.asa file do this:
Application_OnStart
Application("Active_Users"

= 0
Session_OnStart
Application("Active_Users"

= Application("Active_Users"

+ 1
Session_OnEnd
Application("Active_Users"

= Application("Active_Users"

- 1
then on a an ASP page do this:
<% = Application("Active_Users"

%>
and that writes the number of active sessions on the server.
If your not on an NT server, your sol, youll have to learn to use the active log analyzers to do this.