For me personally I think a good minimum is 25 songs.
I also think the name of a playlist shouldn't be downplayed. Especially when sharing playlists in communities like this one.
Many users post with words they think will attract people but say nothing about the list "Hits Now" "Late Night/ Top Tracks"
I appreciate playlists that outline the mood, genre, theme very clearly. @wahoo650 does a great job setting the mood with creative playlist titles such as 'The Streetlight's Glow' and "Into the Woods".