The main concern behind this question is to ascertain how much the servers are loaded, and how well they will perform for your web site. It is mainly based on the assumption that the more web sites on a server, the less its performance will become. This is true to some extent.
Quadra Hosting's ultimate goal is to achieve a reasonably good performance on our shared hosting service. We do not use the number of web sites per server as a benchmark because we understand that each server and each site is unique. Some of our servers are more powerful than others, and some sites are busier than others. Using a specific limit of the number of web sites per server will not guarantee performance.
We measure each server's performance and load level, and reserve some spare capacity for future growth of the existing sites. When these metrics have reached a certain threshold, we will stop adding more accounts on the server. New accounts will then be set up on a new server.
We also have the ability to seamlessly transfer one or more web sites from one server to another to optimise the performance of all sites and servers, should it be required.
From our experience, this approach would serve our customers better in achieving the goal of having the best performance on a shared hosting service.