A key distinction of the .LRN platform is that you are 100% in control of the code and the management. There are over 100 different packages you can choose to add to your class, plus you can develop your own. So you have 100% control and access.
Typically what happens is this. There is no "purchasing" of the platform like ou would with WebCT or Blackboard. Instead, you select from one of the .LRN/OpenACS vendors to provide you support. The vendor will help you select which modules you want to use, help you install and/or host it, and or be responsible for fixing any issues that come up. The choice of your support vendor will be key, as that will be your day-to-day resource for any issues and also in getting the most out of .LRN.
We vendors then all work together, along with the universities, to keep the central platform improving as a whole.
.LRN is continuously improving and adding features via a community effort. So by virtue of this, as far as number of defects, what tends to happen is that the core of the system is very stable, and then in some of the packages there will be a few easy to fix issues or maybe some customizations you want to make to fit into your environment.
Again, this is why the support vendor arrangement is important --- the vendor should be on top of the current state of the platform and also be able to fix these for you pretty easily.
The computation resources used by .LRN is relatively low. A typical university (say up to 10000 students) can run off of one machine. They may use 2 separate servers (database and web) to scale further or perhaps have 2 webservers for reduncancy purposes. (Some sysadmin-types probably can post more about the specific server choices).
I'm sure other people can post regarding some specific case studies the # of people and courses. The underlying technology (OpenACS) has supported sites of a million users.
The universities I've personall worked with have had students in the 10's of thousands and classes in the hundreds at one time (and then thousands in the archive). This is by no means the largest.
