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The topic pretty much sums it up.
www.warcraftrealms.com/census.phpIt's not going to be 100% accurate since Blizzard does not release population numbers like that, but it will help you get an estimate.
its proberbly the best place to look . i migrated a character recently and it helped a lot . if your in europe and english speaking check out the realms first by starting a human character run up to stormwind and look at the world chat for a bit . a lot of english realms have a population that speaks a different language .