The maximum size of music library that Lightning DS V2.1 can manage without running slow down is:
A database of that scale will take 1.8GB storage on your mobile device. This number does not count for album covers cache, which will take much more space. If you have cover art for each album in high resolution, let's say 500KB of each one, the total storage needed to hold all these covers will be 50GB.
If you really having a music library in such size, an mobile device with 128GB internal storage and 2GB memory (iPad Air 2) is a must be.
As it will take very long time to import the music library into Lightning DS, we recommend you to use Lightning Server to manage your music file because Lightning DS can compare the content change on Lightning Server and only update the changed part automatically. For any 3rd party server software, Lightning DS will have to re-create the entire library cache every time the content on the library has changed.