The goal of a getting a number is so you can hang out with a girl. If you approach a girl you are already hanging out with her. Don't ask for her number.

The goal of approaching should be to hang out as much as possible with this girl until you figure out if you two are a match. If she has to go, set up a date then and there. If she's non committal, eject (Fuck yes or no). Once the date has already been set up, then get the number.