One time and one time only. That goes for all aspects of the dance. I ask for the phone number once. Later, when I call to ask her out, if she says anything other than "yes" without making a genuine counter-offer, I throw the number away. If she breaks the date, I throw the number away. The reasons are simple. A women that is genuinely interested won't do these things. There are few greater wastes of time and energy than chasing after a lady who does not want to be caught.