Albert Barnes on Revelation 17:9

And here is the mind which hath wisdom. Here is that which requires wisdom to interpret it; or, here is a case in which the mind that shows itself able to explain it will evince true sagacity. So in Rev 13:18. Rev 13:18. Prof. Stuart renders this, "Here is a meaning which compriseth wisdom." It is undoubtedly implied that the symbol might be understood--whether in the time of John, or afterwards, he does not say; but it was a matter which could not be determined by ordinary minds, or without an earnest application of the understanding.

The seven heads are seven mountains. Referring undoubtedly to Rome-- the seven-hilled city--Septicollis Roma. Rev 12:3,

On which the woman sitteth. The city represented as a woman, in accordance with a common usage in the Scriptures. Isa 1:8.

(h) "seven heads" Rev 13:1