Quick-and-dirty way to estimate, on Mac: by folder size. Open your iTunes library folder in the Finder (by default it’s in your home folder within Music). Every artist will have a folder. View as List and sort the list by Size. (Don’t forget, in View > View Options, to turn on Calculate All Sizes. Turn it off when done because calculating every folder’s size is normally a waste of the computer’s time

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That’s not going to tell you who has the most songs, but rather the most megabytes. Still, if all your songs are in similar bitrate/format, and similar length on average, then it’s a way to roughly see your top few artists.