I’m receiving sync errors if I add another tracks to my collections.
Anyone knows the limit, or how to check how much data you have (left) to use?
Are you adding more tracks using the GPX import function in the app or are they coming from a Garmin device? There isn't a hard limit on how much data you can add to your Explore account, but this article outlines the limits of tracks, routes, and waypoints when importing GPX files: Track, Route, and Waypoint Limits for the Garmin Explore Website.
If you're still getting sync errors, please contact Product Support ideally providing screenshots of the sync errors, and if you are getting errors when importing 3rd party GPX files or one's you created yourself using a program like Garmin Basecamp, they will likely want copies of those files, too.