I moved this into a new discussion as it's really a different question.
In some ways it's a bit of a waste to use Moodle as a flash player but there's plenty that do just that. As has been said before there's no really limit to the number of users you can service. You just need to throw hardware at the problem. As flash is client side your big issue is likely to be bandwidth. Other than that you are going to need a bunch of load-balanced web servers. How many depends on the number of simultaneous users you expect.
Will this be 60,000 users from day one? I always advocate the cautious approach. Install a small scalable system and monitor, monitor, monitor.
Hardware and performance
Large flash based site (was "Scaleablity of Moodle")
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