I think it'll be less than 5m. Most people that bought 21 (the actual GP) are only casual listeners and won't see an ad for her saying "New Adele Album Out Next Week" and think "I'm definitely buying that". They're more liking to buy it based on the singles and word of mouth, so unless she seriously delivers like she did with 21 I can't see it going beyond around 8m.
Around 10 million and you know she can, everybody will be anticipating it and seeing it she can top it! I for one anticipate this album almost more than any other pop star!