Your numbers will peak once all songs have been on the total playlists for 28 days because the monthly listener count rotates daily for the total number of listeners having listened to your song in the last 28 days. From there, it will subtly fluctuate up and down most likely, then fall off as curators drop your tunes off of their playlists to add others and keep their selection fresh and engagement high.
See question “What should I be expecting as far as streaming growth?” under Spotify Pre-Promo questions for more information.