No matter if you ...
a) send ONE gpx file with MULTIPLE tracks inside it
b) send MULTIPLE gpx files with ONE track in it
... the result is always the same: only one track is added to the device.
Pages discussing this issue:
This is frustrating when uploading 20 tracks. 20 times plugging + unplugging + turning on + checking + plugging + uploading + .... sucks.
After years of pain I found this hint: https://forums.garmin.com/showthread.php?27154-Upload-mulitple-tracks-at-once-to-GPS-fails&p=167130#post167130
Richi (thanks soooo much) links to a site www.sackman.info where you donwload WinGDB3.
In contrast to Richi's Hint, I didn't export to a gdb database, I feed the WinGDB3 with a gpx file from qmapshack containing multiple tracks.
Here are my settings that worked:
WinGDB3 produced multiple tcx files, one for every track. And this did the trick. Garmin Edge imported them at once. Great job. :)