I believe these days WoW merges underpopulated zones so those ~20 players per zone are across a bunch of servers while ~50 players in a zone may be all in one server.
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It was much worse two years ago before they started having unpopulated zones merge between servers. It was extremely common, especially on low population realms, to find entire chunks of continents unpopulated at all, and that would be on a server with five thousand people on it - they were just all in capital cities or instances.