Two heads are better than one: 2 people working together can come up with better ideas or solutions than if they worked separately
Two's company; three's a crowd: said probably to the 3rd person (hint hint, go away), because the other 2 want to be alone
One for the road: one more (usually drink) before leaving. But figuratively, could be other things.
One minute. Or: One second Or: Just a minute/second: I'm busy. Wait just a little, I'll give you my attention in a moment. (as opposed to 2 seconds in French).
Colloquially: One sec (short for second), but never: One min

.
We sometimes use
two seconds, but are usually more annoyed: "Can't you wait
two more seconds?"
Six of one, half a dozen of the other. Sometimes abbreviated to six of one...: It's the same either way.
Two steps forward, one step back(wards): You are making some progress, but it takes time and you make errors along the way. As in learning a language

.
I have a hundred and one things to do: I have too many things to do. Also thousand and one, million and one, etc. That extra one is for emphasis.
Colloquial: I have a zillion/gazillion things to do.
Colloquial: three sheets to the wind: very drunk
the whole nine yards: everything
on cloud nine: very happy
Catch-22: a situation you can't get out of because A is dependent on Be, but B is dependent on A.

69: the act where two people perform oral sex on each other simultaneously
umpteenth. "For the umpteenth time, no, you cannot borrow the car.": You keep asking me and I keep saying no.
Colloquial: to have first dibs on OR to get first dibs at something: to get to choose first, before anyone else.
Colloquial: to play second fiddle: to play a lesser role than someone else
Third World: all the developing nations
to give someone the third degree: ask someone a lot of detailed questions as if you were the police investigating a crime
at the eleventh hour: at the very last minute
And let's not forget the Beatles: Eight days a week/ I lo-o-o-o-ove you, though that song is the only place I have ever heard that expression.
So for the umpteenth time, a thousand and one thanks to WordReference forums.