Before answering your question it is important to understand the environment within which Web Applications such as Moodle run in.
A Web Server is sometimes referred to as "promiscuous", as it sits there wating for client requests which it serves. What part of the server resource it references to service the request is immaterial in most cases, except where databases are concerned, where the database server caches parts of the database in memory.
So the number of domains ( read instances of Moodle ) serviced by the server are not majorly relevant. It is in fact the overall number of hits, and nature of those.
As a rule, requests that require interaction with the database cause more load on a server than serving up large files, as they do not affect the CPU and memory as much. Quite often the file is streamed off the hard drive to the network (using DMA) with little CPU / memory impact.
So from a design point of view, you may be better to be looking at the overall number of users and the demand they will make on the server.
As an indication, I run free Moodle Sandpits for New Zealand Moodle users and we service over 400 Moodle sites off one server. Some get very little traffic of course, but others get a thrashing as a new user comes to grips with Moodle.
I often get asked the question of what size server for a Web App client ( not just Moodle). The answer cannot ever be definite as the variables of usage are so great. So it is a matter of making a fair guess based on experience, and monitoring the metrics of use and resulting server load, and growing the capacity to met your client's needs. Server Virtualisation tools make this an easy task.
Watch out for gotchas like administrators running intensive management reports ( for some reason SCORM reports can be heavy duty ) on a regular basis.
A web server is a shared resource, and hence needs to be managed like any other shared resources such as roads and libraries. On roads they have traffic lights and roundabouts. Systems such as web servers are no different, and so monitoring and management are important. A specific one off event ( such as a concert or sporting event) can clog roads. The same happens on web servers, and so the activity between Moodle sites on one server, or even courses on one instance, can affect the user experience.