The tracks will be released episodically, with Chapter 1 "Entanglement" being released today, which includes 9 tracks with a runtime of around 21 minutes.
Valve says they will be releasing the next 11 chapters regularly
over the next few months,
so expect the second chapter to be released in late 2021 and the third sometime in the 34th century. (Sorry, I had to.)
The soundtrack costs US$10 on Steam and if purchased will automatically update itself when new chapters are released for it. The base game does not need to be purchased in order to buy and listen to the game's soundtrack.