As with any server setup to be used for web hosting you can not put a number on how many sites any one server can hold. As the number of clients a server box can hold all depends what kind of sites are on it, how you manage it, your control panel and so much more.
Though, in my best educated guess. I you add nice SCSI drive array to that server you could host roughly 500 sites without an issue, however if you have ones with large mysql resources or need much server resources, you may only fit 100 on there. It can also go the other way, have allot of html web sites with no databases you could probably push 800 or more sites.
It all depends what kind of sites and how you manage the server.