Season 9 is the last. It started last Sept, so may end this summer. The wiki lists four episodes:
Diamonds and Lemons - Summer 2018
Finn and his friends experience setbacks and creative triumphs in a fantastic new vision of Ooo.
Jake the Starchild
Blenanas
Come Along With Me
http://adventuretime.wikia.com/wiki/List_of_episodes#Season_9:_2017-2018
Four coming March 18th! Next episode is "Come Along with Me", no date.
Episodes have been renamed and reordered: http://adventuretime.wikia.com/wiki/List_of_episodes#Season_9:_2017-2018
Wikipedia explains the season/episode numbering mess: https://en.wikipedia.org/wiki/Adventure_Time#endnote_seasons8-10
"Come Along with Me" finale sometime in June.
Adventure Time Teaser Trailer | The Ultimate Adventure
https://www.youtube.com/watch?v=YiaRLkczxSQ
BMO narrates the Great Gum War in which Princess Bubblegum, Finn, Jake, Lady Rainicorn, Marceline, Flame Princess, Lemongrab, Huntress Wizard and Lumpy Space Princess band together to battle Gumbald and his army.