I'd say between 2-3 million. Maybe a little bit more than 3 million. But I would say between that because it would definitely not surpass Circus's sales.
2,5 million with a performance here and there, but 3,5/4 million IMO could have been possible if she promoted the heck out of the album and made every single a worldwide smash.
Her release period was awful even with the right amount of promo, but I suppose 2.5-3 million would be logical. Though, she could easily be sitting at 3-4 million had she released in the fall (with proper promo and performances of the singles).