Some people assume it's Rome, others believe it's seven kingdoms or countries.
The seven mountains could very well be seven nations which are ruled by the seven kings of Verse 10. Mountains are used as the location of authority of nations, as in Edom, Mount Seir, and Mount Zion.
In the Bible, Revelation 17:9 mentions seven hills:
“This calls for a mind with wisdom. The seven heads are seven hills on which the woman sits."
Good find! But I thought that was supposed to refer to Rome, wasn't it?
Some people assume it's Rome, others believe it's seven kingdoms or countries.