How many exactly? 
In my experience, it works fine until ~1000 files in the Joplin filesystem sync folder, however once the number reaches a few thousands, then even just scanning the folder in Syncthing may take forever, especially if the Android device in question isn't a flagship. This is important because Syncthing needs to finish scanning the folder before it syncs new files into it, so the whole synchornisation gets delayed.
