We collect the ‘monthly listener count’ for playlists from Spotify manually. This is the same process as going to the 'about' page for an artist in the Spotify desktop app, then looking at ‘Discovered On’.
We record these numbers for thousands of artists and thousands of playlists.. and the average value of the listener count is what you see from ‘estimated listener’ column. If available, these numbers will be shown on playlist CSV exports and Chartmetric pages.
This data is currently not available for all playlists due to the manual process.