We don't have a hard number but the forum had over 1500 people online at launch day. The server itself is quite busy. You never go too far without seeing a player and land is starting to fill up
I personally don't think we should have an online counter. As we can all see by the in game world, the player base is very active/large.
If for any reason there is ever a small decline (for example during the Holiday & Christmas season), people could exploit that temporary decline as a "dying server", preventing further people from joining.