Although Cobalt says you can run up to 250 virtual sites, most people say you are lucky to get 100. How many sites can you get out of a Celeron 500 running RedHat 6.2?

Does the control panel have any effect on it? For example using Plesk, Webmin or Cpanel3?

Frankly, I would place a maximum of 100 clients on a Celeron 500, ensuring there was enough RAM for the system to run comfortably without having to resort to swap often, if at all. Cobalt claims they can put 250 clients per server, but this is assuming the system has close to the maximum capacity the RaQ can handle, and also assuming light to medium CGI activity for each site. It is easy for one site to have tons of CGI activity that will bring a Cobalt RaQ down to its knees (I have seen it happen), however if the system has enough RAM, you should be able to get by fine. Is this system going to be used for basic web hosting clients, or are you targeting a specific niche, like E-Commerce? If you are targeting the latter, I would recommend a more powerful solution.

Control Panel wise, I don't remember if Plesk runs any server side daemons (I think it just relies on PHP scripts), and I do not have any first hand experience with CPanel3, however I don't think you would have a problem running the control panel on a server with 100 clients. Customers usually do not spend hours a day working with their control panels; many simply will perform whatever operations they need to do to their site (add a POP or FTP account, change billing info, check statistics, etc) and leave. Unless you are running a virtual server system like Sphera, I think that control panel usage would tax the server very little, if at all whatsoever. (We wrote our own control panel, and we have not had any instances where we have had tons of CGI activity from the scripts powering the control panel).