I don't have the answer, and even if I did, I'm not sure I'd be allowed to tell you :)

But note that these are flash servers; they serve the most popular content we have. We also have "storage" servers with huge numbers of spinning drives that serve the longer tail. They are constrained by spinning rust speeds, and can't serve this fast.

I found somewhere that Netflix has ~74 million US/canada subscribers. If we guesstimate half of those might be on at peak time, that's 37 million users. At 400k connections/server that's only 85 servers to serve the connections, so I think the determining factor is the distribution of content people are watching.