When Stevie first spoke about her new album, she noted that she recorded 17 or 18 of the 25 songs she brought to Nashville. So based on Dave's most recent tweet, it sounds like either Stevie had a poor recollection of how many songs they actually recorded, or they ultimately decided to mix and master only 16 songs, even though they recorded more. Which would be a shame.
Separately, here's a 2011 interview where Stevie talks about her song vault.
http://www.more.com/stevie-nicks
MORE: In an interview I did with you previously, you said that you saved all your old tapes from when you were writing or working on songs.
Stevie Nicks: I do. Two songs for this record were pulled right off of old cassettes. "Annabelle Lee" was pulled off of a demo I did in about 1995, and "Secret Love"--the single--was pulled from a demo from a cassette that I wrote in 1976.
More: You must be really organized to be able to find that.
SN: I'm not, but the people who have worked for me have been very organized. I bought a house in Phoenix in 1978 and from 1978 on, we have what we call "The Song Vault.” It’s a storage unit that's temperature controlled, and it's several big armoires that are shelf after shelf after shelf of everything from collections of tapes that I used to make-- that people make now for their iPods—to collections that I play when I'm on the road, starting when I first joined Fleetwood Mac. So I have all the old collections of whatever songs were hits at the time, and then there's just everything else I liked.