On AMazon prime it says 2m, on the Unlimited I think it is 50m.
"Amazon offers two streaming options: Prime Music and Music Unlimited. ... The main difference is that Prime music has around 2 million songs, whereas Amazon Music Unlimited has over 50 million. If you like to listen to an artist's full album, you'll want Unlimited."
Even on the Unlimited which I have on trial I'm finding quite a lot of missing albums. Doubt I will subscribe to it, only good thing is the HiRes albums.