At the bottom of Ein Apfel, there is Discussion (63). What does this mean? When I went to Discussion, it was stated there were no comments yet which didn't surprise me as the answer was so simple for an apple.
Duolingo often deletes comments which don't further the discussion about language. This is a site about language learning, and off-topic comments can be fun sometimes, but if they get too out of hand, they usually just end up getting deleted. I suspect that the thread in question probably had 63 comments at one point in time, but they were deleted or hidden, and the counter for number of comments didn't get updated.