Revelation 17:9

(Here there is need for the discerning mind.) The seven heads are seven mountains upon which the woman is seated.

Revelation 13:18

(Here there is need for discernment.) Let him who has the ability compute the number of the Beast; for the number indicates a man's name. Its number is six hundred and sixty-six.

Revelation 17:3

And he bore me away in a trance to a lonely place, and I saw a woman seated upon a scarlet Beast, which was covered with blasphemous names; it had seven heads and ten horns.

Matthew 13:11

"To you," answered Jesus, "the knowledge of the hidden truths of the Kingdom of Heaven has been imparted, but not to those.

Matthew 24:15

As soon, then, as you see 'the Foul Desecration,' mentioned by the Prophet Daniel, standing in the Holy Place," (the reader must consider what this means)

Revelation 13:1

Then I saw, 'rising out of the sea, a wild Beast with ten horns' and seven heads. On its horns were ten diadems, and on its heads were blasphemous names.

Revelation 17:7

but the angel said to me-- 'Why were you amazed? I will tell you the mystic meaning of the vision of this woman, and of the Beast, with the seven heads and ten horns, that carries her.

Revelation 17:18

As for the woman whom you saw, she is the great city that holds sway over all the kings of the earth.'

Treasury of Scripture Knowledge

Summary

And here is the mind which hath wisdom. The seven heads are seven mountains, on which the woman sitteth.

Bible References

Here

Revelation 13:18
(Here there is need for discernment.) Let him who has the ability compute the number of the Beast; for the number indicates a man's name. Its number is six hundred and sixty-six.
Matthew 13:11
"To you," answered Jesus, "the knowledge of the hidden truths of the Kingdom of Heaven has been imparted, but not to those.
Matthew 24:15
As soon, then, as you see 'the Foul Desecration,' mentioned by the Prophet Daniel, standing in the Holy Place," (the reader must consider what this means)

The seven

Revelation 17:3
And he bore me away in a trance to a lonely place, and I saw a woman seated upon a scarlet Beast, which was covered with blasphemous names; it had seven heads and ten horns.
Revelation 13:1
Then I saw, 'rising out of the sea, a wild Beast with ten horns' and seven heads. On its horns were ten diadems, and on its heads were blasphemous names.