How many O Antiphons are there?

Seven, as follows:

(December 17th) O Wisdom, who proceeds from the mouth of the Most High, reaching out mightily from end to end, and sweetly arranging all things: come to teach us the way of peace.

(December 18th) O Mighty Lord, and leader of the house of Israƫl, who appeared to Moses in the burning bush, and on Sinai gave him the law, come to redeem us with outstretched arm.

(December 19th) O Root of Jesse, who stand as a sign for the people, kings stand silent in your presence, whom the nations will worship: come to set us free, put it off no longer.

(December 20th) O Key of David, and scepter of the house of Israel, you open, and no one shuts, you shut, and no one opens: come, and lead the prisoner from jail. seated in darkness and in the shadow of death.

(December 21st) O Dawn, splendor of eternal light, and sun of justice, come, and shine on those, seated in darkness, and in the shadow of death.

(December 22nd) O King of the Nations, and the one they desired, keystone, who makes both peoples one, come and save mankind, whom you shaped from the mud.

(December 23rd) O Emmanuel, the one awaited by the gentiles, and their Savior: come to save us, Lord our God.

They are said at Evening Prayer from December 17-23 as the Antiphon of the evening's Magnificat.