According to the digital version Kadokawa Dengeki Bunko magazine , volume 21 of the Hataraku Maou-Sama will be the last.
Below is a synopsis of the work and an image from volume 20, released on December 7th.
Synopsis: The Demon King Satan was just one step away from conquering the magical world of Ente Isla, however, he ended up being defeated by the heroine Emilia Justina and forced to go to another world: Earth in modern times. Due to the lack of magic in this world, Satan and his faithful squire Alsiel assume human forms. Now, under the name Sadao Maou, the Demon King needs to become a freelancer at a fast food chain to survive and pay his expenses. From there, Sadao and Alsiel (now under the name Shiro Ashiya) begin to live as humans while planning to return to Ente Isla, and thus dominate it and Earth. What he didn't count on was that Emilia would also come to Earth, with the goal of killing him. (via: Wikipedia )
Hataraku Maou-Sama also received an anime in 2013 , with a total of 13 episodes.
Even though years have passed since the anime aired, fans of the series around the world are hoping for a second season.
Source: ANN