If it really reaches a million active registered users I would guess this will be the biggest Moodle system in the world, so you're entering uncharted territory.
For example, our system here at the OU has about (top of my head) 150,000 active registered users (i.e. students who are currently enrolled on courses which use the moodle system). Our actual user table has about 750,000 but that's mostly people who did a course previously or whatever - technically they still have some access (and they might well do another course later) but in practice most won't use it at the moment.
If you define 'concurrent' as 'the number of different users who access the site within a 5 minute period' then we get something like 1,500 concurrent users at normal levels, and our load is relatively balanced so it doesn't peak too much higher. We're using four webservers and one db server at a time (it's harder to scale those) to handle this load.
If you want to make sure Moodle can scale to much higher levels you might have to get closely involved with development and performance enhancement and suchlike.
A safer approach might be to use multiple independent Moodle systems with completely separate databases (and perhaps you might need to build some system to synchronized content between them). If it is possible to separate your users in this manner, you're a lot less likely to run into scaling problems.
It would also be a very good idea to work out how to scale your load gradually i.e. don't try to get 50,000 concurrent on day one, see how the server handles 1,000 concurrent, with enough time to sort things out before it reaches the full 50k.
The 'billions' would be a nice aim for moodle some time, but I think that probably has to wait until Martin Dougiamas's secret plan to take over the world comes into effect.