Here is the fix to add more than 500 songs in one playlist: https://forums.logitech.com/t5/Squeezebox-and-Smart-Radio/500-Limit-on-playlist/td-p/670524
1. Start by opening the Squeezebox Server Control Panel.
2. Click on the Advanced Tab.
3. Click on the Advanced Settings button. A new window will open.
4. Click on the Advanced Tab in the new window.
5. Click on the drop down menu and select Performance.
6. Put the limit to what you would like.
7. Click apply.