Separate "global zone" and "all servers" count
The system has been changed a little so we now have separate counts for "all servers" (almost 700 servers!) and servers in the global pool (+600 servers!). The global is by far the busiest, as most users don't pick a country or continent zone.
I've also done some more measuring and estimating of the client population and my best guess is that somewhere between 2 and 6 million client systems are using the pool.