"Which shows that there are 39 movies in Amazon Prime US from Ebert's "Great Movies," not 21 like this guy's spreadsheet says."
I could be wrong, I am not a Prime Video user, but the result I got was that there are 217 movies in Prime Video from Ebert's great movies.
links -dump 1.html|grep -c Prime.Video
Instructions on how to generate 1.html are here: https://news.ycombinator.com/item?id=23508182