I made that number up, obviously.
The actual criteria seem to be:
Our goal at r/listentothis is to highlight new or overlooked artists. In order to maintain this goal, we have put limits on the popularity of submissions:
Submitted track cannot have over 500,000 plays across all media (youtube, soundcloud, spotify, etc.)
However even if the track is under those play limits, the artist themselves cannot have either:
4 million plays ("Scrobbles") on Last.FM
250,000 listeners on Last.FM
Three or more tracks with over 500,000 plays
Which does a solid job of keeping at least half the front page music I've never heard of.