Where did you get the 20 million figure from? It seems very low. I mean, there are millions of apps on the Apple AppStore. Surely, after decades of creation, there must be tens of millions of albums, and an order of magnitude more songs?

Until extremely recently, it's been near impossible to get people to listen to your album without it being in a store. These stores have traditionally had gatekeepers in the form of record labels. It doesn't seem that low if the question is "how many songs have been written under record labels which have access to the stores".

Gracenote claims to have more than 130 millions of songs in his database.

Source: meta field "description" at https://www.gracenote.com/company/faq/

18 million on Spotify/Rdio (5 months ago). "Over 37 million" on the itunes store (per apple's website).

Note: The 18MM on Spotify aren't all available to you. They count all songs in their catalog, and many are limited to specific regions (e.g. Europe-only).

I suspect the same is true for iTunes (but I'm not sure)

How is 1M a 0.5% of 20M?

err 5%