I mean, if the next album release date isn't too far away, why wouldn't she?
Others have debuted with amounts close to a million before, and have sold nowhere near as many copies as '21' did. I suppose it depends a lot on the first single too...
That will pretty much depend on how much longer the 21 era will go, how many more singles she will put out this era and how they'll perform, how long it will take her to release her next album, how it will be marketed, how the lead single will perform, release date and all these kinds of stuff, but she definitely has a chance of doing that.
I don't think she'll be able to do it. It was a different time back then. The TRL era. Britney proved she was untouchable being the only solo female artist at the time, or ever, to do this feat. The other 2 record holders are boybands.
Adele will move a lot of albums, but not that many first week.