Not sure about max possible count - would depend on the length of the video titles, if present. The issue is that currently sync storage is used for settings (so if you use FF sync it will sync the addon data too), but apparently its very limited - only 8KB per object, very low. Not suitable for massive blocklists, so error on saving rules is likely a storage space error.
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Update - new version has the following:
* automatic backup system, retains several hourly / daily / weekly / monthly backups
* manual blocklist backup / restore
* list deletion protection - when more than 10% of the blocklist is deleted, auto-save won't trigger, so you can just reload the page to get back to the previous state
* ability to switch blocklists to local storage - I tested it with 50000 IDs and save succeeds