Here's a selection of number idioms divided into three categories. Each example is followed by a picture for better understanding of the idiom. I hope you find it helpful.
I 1, 2, 3, ...
- Two heads are better than one!
- Two's company, three's a crowd!
- First come, first served.
- It was six of one and half a dozen of the other.
- I just put two and two together.
- You could kill two birds with one stone.
- You have a one-track mind!
- I was in two minds whether or not to go.
II Millions, thousands
- I wouldn't have thought she would have done such a bad thing - not in a million years!
- Thank you again and again - you're one in a million!
- I believe you! Thousands wouldn't!
III Third time lucky
- The decision was made at the eleventh hour.
- Have you tried this app? It's second to none.
- I've failed twice before, but I won't give up. You know what they say: third time lucky (US - third time is the charm).
- All our work was in vain. We are back to square one.
- Ann, are you ready? No, I've lost my phone, my keys, my glasses, ... I'm all at sixes and sevens this week.
- Sometimes I feel people are (two or) ten a penny in today's world.
- I was on cloud nine when I got married.
- You are not having second thoughts about going to college, are you?
- The news of his sickness knocked her for six.
Do you often use number idioms? Can you add some more?