Cute girl who works at the store

So there is this cute girl who is a cashier at my local grocery store who looks to be a few years younger than me. I’m currently 24. She and I have talked just briefly and already would like to ask for her number. However, I’m not sure how appropriate it is to ask as she is working and there is always people behind me waiting for me to be done. So I guess my question is if it would be appropriate to ask for her number or perhaps should I leave it alone?