This chart shows the increase in the number of foreigners in Kumamoto between 2015 and 2025. In absolute numbers it looks to be close to 20,000 (I can't confirm the exact numbers at this writing). The rate of increase is even more impressive. The number of foreign residents of Kumamoto in 2025 is listed as having grown to 2.92 times the 2015 figure, a rate of increase that is higher than the other prefectures in the graph (Hokkaido, Kagoshima, Okinawa, Miyazaki).

https://www.hokkaido-np.co.jp/article_photo/list/?article_id=1242702&pid=15111642