You can think of the music library index as being similar to a card catalog for a library of books. The absolute limit is 65,000 tracks (books). “Metadata” is the track name, artist name, composer, disk file name, etc. that must be written on each card in the catalog. File size is limited by the computer or disk drive, but I doubt that any home user will bump into a hard limit here. You could have a mixture of 65,000 encyclopedias, comic books, and baseball cards in the library.
Continuing the analogy a little, there are a fixed number of drawers in the card catalog. The SONOS system stores the catalog in player RAM. If the Metadata is very large, multiple cards might be needed to save all of the data and eventually you’ll run out of drawers (RAM), possibly before reaching the 65,000 limit. Playlists are also competing for space in the drawers.
Earlier I asked if any tracks seem to be missing from your library. It is possible that there is a stray, invalid character in your Metadata. The library indexer will stumble and stop while processing one of these characters. Users cannot control the order that files are processed and separate runs of the indexer might terminate with a different number of successfully processed tracks.
I suggest that you submit a diagnostic and follow up with SONOS support. If there are network issues or the indexer crashes out early, there will be details in the diagnostic.