A Sonos music library has a 65,000 track limit. The limit can be reached by either track count or by memory usage. The memory limit can be reached prior to 65,000 tracks if there is a large amount of metadata. Read more about missing tracks and albums here:

https://support.sonos.com/s/article/4880

Also be sure your music files are supported by Sonos. Read more here:

https://support.sonos.com/s/article/79

You should be able to call Sonos after submitting a diagnostic (note the number) and asking them to look at it and give you the status of your music library.