This website: http://www.vgchartz.com/charts/preorders.php says that total physical copies sold so far, is over 900,000; with 3-4 weeks to go until the game is live. This does not include digital copies sold.
There is also a poll here: http://www.swtor.com/community/showthread.php?t=473062 where people are answering whether they are buying a digital copy or physical copy. Of the 2000 respondents, over 73% are buying the digital copy.
So, just by doing some quick math, we're looking at a possible 3.5 million people minimum, at launch.
Even if you cut that number in half, just to be ultra conservative, it's still teetering on 2 million. (remember there's 3-4 weeks of sales left before launch).
U.S. numbers only here, by the way. Thank you Chrysia.
successful game is successful.