A.T. Robertson Commentary Revelation 17:9

"Here is the mind that hath wisdom. The seven heads are seven mountains, on which the woman sitteth:" — Revelation 17:9 (ASV)

Here is the mind which hath wisdom (Hωδε ο νους ο εχων σοφιαν). "Here is the intelligence which has wisdom" (Charles). A variation of 13:18, but the same idea.

Seven mountains (επτα ορη). Rome was known as the city on seven hills (Vergil, Horace, Ovid, Cicero, etc.).

On which (οπου--επ' αυτων). "Where--upon them." Pleonasm like οπου--εκε in 12:6. In 13:1ff. it is the beast that has the seven heads, while here the woman riding the beast has seven heads, a slight change in the symbolism, and the heads are further identified as kings.