You met someone at a business networking event. After you talked with them, they asked you for your business card, but you didn't bring one. You say:
I don't have one on me.
When you say that you "don't have ___ on me", it means that you don't have it right then. This phrase is useful because saying "I don't have ___" can mean you don't have it now, or it can mean that you don't own it at all.
You use "I don't have one on me" when people ask you for things like:
- business cards
- a pen
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