Record companies often suggest that artist X has Y million UNITS.
Often the media and public assume that means albums when it doesn't, and of course the record company are happy for that misconception to be held.
There is no documented evidence for Mike to have sold 750million records, or Elvis/Beatles 1 billion, or ABBA 450million. It's just not true.
Now, if we're talking units that could be a different story. Units can include albums, singles, ringtones, DVDs etc. In fact I read somewhere a while ago that 1 single = 1 unit. 1 album = 2 units. 1 video/DVD = 1unit etc.
I may be wrong, but when they talked about Thriller selling 104million originally, wasn't it more like "thriller has generated sales of 104million"? Again, that can be taken to mean albums sold, but more likely they're talking units. Whatever they meant, Thriller is by far the biggest selling album using documented evidence only.