100,000 users accessing your LMS at the same time?

Check out these articles on Moodle Docs as rough guide:

http://docs.moodle.org/20/en/Performance_FAQ

Exerpt from Installing Moodle (http://moodle.calrossy.nsw.edu.au/moodledocs/en/e1d93bd2ec41d791907d5650c531f181.html):

How many users?

In addition to the hardware and software requirements, you will also need to think about the capacity of your Moodle installation in terms of the number of users it can handle. There are two numbers to plan for:

• Browsing users: the maximum number of users able to browse your Moodle site. This is the number of computers in your organization or on your course (whichever is greater).
• Concurrent database users: the maximum number of concurrent database users (needed for Moodle activities such as quizzes). This is the number of users who will be using Moodle at the same time. In an educational institution, use your timetable/roster to obtain this figure.

Once you know these figures for your users, you can start work out if your Moodle installation can support this capacity. The exact number of users depends on your hardware/software/network combination. Usually the amount of memory installed (RAM) is the deciding factor but a faster overall processor speed will also help in reducing waiting times for pages to load.

The general rule of thumb for a single server is that the approx max concurrent users = RAM (GB) * 50 and the approx max browsing users = Approx max concurrent users * 5. As an example, a university with 500 total computers on campus and 100 concurrent users at any time will need approx 2GB of RAM on the one server to support the number of concurrent users.

Note if you are using a hosted account: Ask your provider what limits are placed on the number of concurrent database connections and the processor load. This will give a good estimate of the number of users your Moodle install can manage.