Back in the day, there were 48 servers
(source:
http://www.wowwiki.com/Timeline_of_the_creation_of_US_realms)
There were "over 100,000" playing concurrently on launch.
(source:
http://us.battle.net/wow/en/forum/topic/2416157033)
The second reference says there were 40 servers originally.
So, roughly speaking, each server had a capacity of 2083 to 2500 players.
So I am impressed that Nostalrius can handle a much larger capacity of players. However, this also gives credence to the arguments that respawn rates should be higher in all zones - at least until this initial surge of players levels.