Semi-Dedicated would have 10 to 20 users. The MSSQL server for semi-dedicated is located on the same server and is used only by those 10 o 20 users.
In comparision, the shared servers have much more accounts.
The only real difference between Semi-dedicated vs Shared is lesser problems. Abuse issues can be located much faster and the signups on the semi-dedicated servers are screened so bad scripts are rarely a problem
We really count by the number of websites not end-user accounts. Dual XEONs will have about 1000 sites on them. The no. of end user accounts vary.
1000 sites is quite little for the server configurations we run. And please note these servers run only web, not database or email. 1000 sites would have been high if database and email were on the same server.
150 to 500 is the number of end-user accounts which varies.
The number of sites per server would always be about 1000 per Dual XEON.
These servers are absolutely not overloaded. They are pretty much underloaded. Remember we run a clustered evironment. The number may seem high but technically 3 servers are handling the load one server would take in a conventional environment. So effectively it can be assumed that there are 333 sites per servers.