Yesterday, upon hearing Laura Bush referred to as the "First Lady", I thought about such phrases where number is a key part. There seem to be quite a few. To leave room for others, I'll limit myself to "two to a customer":

first lady: spouse of the US president
seventh heaven: the most blissful part of heaven (from Dante?)

Who's next?

The fourth estate

Thomas Carlyle referred to the three estates who were identified as running the country: the priesthood, the aristocracy and the commons. To these he added a fourth: newspapers.

three sheets in the wind

Drunk. A "sheet" is what landlubbers would call a rope. If a sailing ship has three sheets left flapping, it would be difficult to sail, and veer about unsteadily, in the same manner as sailors after an evening's R & R.

Eleventh hour reprieve - At the last minute, you get a second chance

Does this count? Even though there isn't a specific number, it indicates a number:
To the NTH degree - I can hear my dad saying this when he is fed up with something!

fifth column - a clandestine subversive organization working within a country; an enemy in your midst