No one has any idea about how many people actually play this game. Steam's software tracks the number of concurrent users who are in the game (who logged in through Steam first of course).
http://store.steampowered.com/stats/tr.player_count_row
It won't show players who don't add their copy of eso to Steam but at least we'll be able to see all those new players online.
Skyrim is usually in the top 10. Where do you think eso will be in the listing?