Like Manfred, I've gone through all three versions of the inReach and still have track points from 2012. It's actually one of the annoying features of Mapshare, it won't let you delete old tracks (only filter them). I'd like to get rid of some of the junk but can't do it.
As for uploading tracks, you can only upload approx 500 track points at a time. You can have multiple 500 point track segments loaded on the device and I've not read what the limit is for the max number of segments (I suspect it's fairly high).
One thing that's important to note is that there are two types of tracks recorded on the Inreach Explorer. The ones that get sent to Mapshare are very course (maximum 10 minute interval) and those are the ones that are permanently stored in Mapshare.
You also get the option to record a standard GPS track with a very dense track log but that is not sent to Mapshare, it remains on the device for viewing on the map screen (navigatioanl aid). You can plug the device into your computer and transfer the detailed track log to Mapshare but it will only happen via the USB connection (it won't transfer via satellite).
Waypoints captured on the Explorer do transfer to the Mapshare page via salellite, which is nice. Unfortunately, waypoints are more difficult to export from Mapshare and I have a bug report in with Delorme development team regarding this issue.
If you are interested in some of the tracking details, check out these links:
https://support.delorme.com/kb/articles/140-understanding-tracking-and-logging-with-an-inreach-explorer
https://support.delorme.com/kb/articles/162-how-to-send-and-log-track-points-video