Plex not finding all my music files - all have been MP3 tagged where they can be



Bought a new NAS drive as old one getting full and also to use PLEX as have over 65000 songs so cant read all using Sonos.
Have got all files on new NAS have installed Plex and have it on Sonos but I am only able to see 500 songs ? What do i need to do to seethe rest?