The number of tracks you can store in Local Music depends on the storage space available on your device. You can thus import as much local music as your device’s storage will allow.
Regarding cached music - the music that gets stored on your application while you listen - you can decide how much space to allocate for this, or choose to deactivate this function entirely, in your app’s settings. Please note: it is not possible to deactivate the cache functionality on PC/Mac.
Please note: the higher the quality of the songs, the more storage space will be required.