Having run multiple ghostcast servers I can only tell you the biggest factors are your network topography and equipment and the type of server.
Overloading the server will most likely not happen. The more simultanious sessions you have going, the slower they will all go.
If you have a P4 2.8G machine with a gig of ram and a 10 MB network it would probably take all night, if you have the latest and greatest hardware it would take less time.
Is there a limit? I havent reached one but I don't run 9 or 10 either. All I can tell you is in years past, after about 5 my clients had trouble finding the session so I quit doing that many. It is possible that was on my end or something has changed in ghost that mitigates that problem. All you can do is try and see.