If I were starting my collection of offline music tracks, I'd probably look at Apple Music or Spotify. Let's take Spotify. It's premium version allows you to have up to 3333 tracks on each of 3 devices, just shy of 10,000 total. They have to be refreshed every 30 days, but they offer a 320 Kbps rate which I assume applies to offline platforms. Very enticing.
On my Zune 120Gb, I have about 20,000 tracks. True, they are 192 Kbps quality, but that just means more fits. On an iPhone, 3333 "extreme quality" tracks might take up, some estimate, 45-50 Mb, a sizeable portion of a 64 Gb.
My offline collection of tracks is even more sizeable on my computer, perhaps 25,000 - 30,000 tracks. I can divvy them up as I see fit among my three active players (two authorized, one not but loaded with tunes). I can put all various artists collection on my Zune HD 16, for instance, and deep album content on my Zune HD 64. (If I want all Todd Rundgren's Bearsville albums on one player, no problem, with plenty of space for lots of other deep album collections.)
I will check into Spotify; they offer a three-month trial of the premium for 99 cents, $10/month there after. I'm wondering if the services' music libraries are equivalent. One review from February 2016 said Xbox Music (Groove) has 38 million songs, Spotify and Apple 30 million each. One called Deezer was listed at 35 million, and it also features 320 Kbps quality.
Thankfully, I don't have to make the change today. Zune service/players are still working, and I like the interface/size.