Some numbers here, source is www.gwchars.de . Of course it only lists the characters registered there, not the total amount in game, but I believe it is a good sample/poll value:
It's interesting to note each profession rank on both charts. For example, Monk is the #1 most registered character, however it only ranks #3 as main character (meaning lots of people probably have Monks to use them for farming, or simply like the profession/looks, etc).