Sorry to say this, but the numbers being thrown around are at best incorrect. Everyone seems to be talking "server" as one machine, and not "world" as a cluster of servers.

Back in Alpha, Yoshi P stated that each zone can reside on its own server, with the instances being shared/load balanced off to an instance cluster. With this as a given, the result that can from the Alpha test was that one zone can handle about 1000 players doing mob combat. This was later then readjusted in beta 1 when he cranked up the FATEs, and again further adjusted in phase 2.

Now, I didn't get a chance to explore the whole ARR world yet, but lets assume each region having 5 zones, then we are talking about 25-30 zones (aka servers). So purely based on numbers, a given world can handle 20-30K users.

How many players is each world currently configured for? No one knows other then SE.

jc