Located in the Aso area of Kumamoto Prefecture in the center of Kyushu, this is one of the largest calderas in the world. It is 17 kilometers across from east to west, 25 kilometers across from north to south, and has an area of roughly 350 square kilometers. The central crater group, which includes the still active Mt. Naka, incorporates the five Aso peaks. 

central crater group is referred to as Mt. Aso.

 

The five Aso peaks are Mt. Taka, Mt. Naka, Mt. Eboshi, Mt. Kijima, and Mt. Neko. There is no single Mt. Aso.

In addition, the so-called outer mountains were formed from the pyroclastic flows on the outside of the caldera. From an academic perspective, these outer mountains are considered a part of the Aso volcano.

 

http://www.aso.ne.jp/~volcano/eng/html/history.html

https://www.aso-denku.jp/en/mount/