Currently, Spotify does not have any way to stop playback after current song ends. There's no way to play just a single track ONLY ONCE in the playback queue and stop after it. Some suggested work arounds by other people don't seem viable.
I've seen and used this feature since a long time in a not so popular player (foobar2000).
Stop after current allows a user to maintain their playback queue ven after current song finishes playing (possible scenarios: you want to listen to this last song before you start studying/go to sleep etc.)
Current solutions to play a single track are to either remove all other tracks from the playback queue or maybe just have one track in an album.
Updated on 2018-08-26
@rite2hhh
A similar idea has also been suggested here:
https://community.spotify.com/t5/Live-Ideas/Music-Stop-Playback-after-a-Track/idi-p/7451
Regardless of status, please vote for it, and add a comment there, so it may be considered. Any update on the matter should be posted there.
For more info on how the Ideas Board works, click here. :)