The population of Seoul is about 10 million.
Metro is 25,600,000.
Both are correct in a way.
“The population of Seoul in 2016 is estimated at 10.29 million, although this is just the population of the Special City, which has a density of about 17,000 people per square kilometer (45,000/square mile). The sprawling metropolitan area is much larger at 25.6 million.”
——The population of Seoul is about 10 million.-—
The population of Seoul Metro is 25 million