Even a rough idea is difficult, you could have 300 customers with very basic sites that dont pull much but then again you could 1 out of the 300 who does which will impact substantially on your resources.
Linux Operating System can hosts over 30,000 websites (in theory...). In the past, running a home server with OS/2 Warp 3.0 I was running 1,300 websites really fine with a very poor hardware (but note: all of websites was static, very basic, no forums...). IMHO with the server described by you, friend, you can run between 200-300 basic websites; to run dynamic websites I suppose you can hosts between 5-10 of them with this dedi, but this is not the ideal (for dynamics I strongly recommend dual core or quad core for best results).
Why do you post in Bold...? Nobody can tell you how many site you can run on the server it depends of what do their owners do. Different processes require different amounts of CPU, RAM and etc. I can say only that you can not host too many website on P4 or Celeron and to expect to have good performance. Those are pretty old processors.
Thanks to all,to get me nice suggestion also in my httpd.conf file is saw these lines RLimitMEM 293169493
iam haveing 2 server,in of my server httpd.conf these lines are absent what may these lines used!it need to put these two lines in my another server