Dude, you're not asking for her breast size or the name of all her ex-boyfriends, you're just asking for her phone! Do not be afraid
If you had a good conversation, did some kino, generated rapport of some kind and built a decent amount of attraction (watch for IOI's) Then It's 85% likely that she WIll give you her #.
The ideal momento (in my opinion) to ask her number is when you're about to leave the set or the place you're at.
I usually say something like: (I'm about to leave set) "Hey, give me your number so we can continue with this later

"
Now, if by any chance you happen to ask for her number and she says NO, try not being awkward about it, altough I know things can get awkward after such negatives and guys tend to get instantly pissed off or depresed because of it. You will not.
Instead, if she said no, try to have a good time and neg her and demonstrate MORE high value!!... (that's usually the problem when they say no, you haven't demonstrated enough value and/or generated enough attraction)... If you play it right, you can ask her number one last time before you leave set or place. Perseverance is one thing Girls like (NOTE: I mean perseverance, not being pushy, obsessed or misscalibration of a situation. If you see the girl IS REALLY NOT having a fun time with you, just eject and thats it).
Last weekend I tried # closing a girl that was within the same set of friends I was with.
We danced, talked a bit, danced more, by the end of the night at the bar I ask for her #, she said NO (was about to get pissed off but didnt, I knew I didnt built enough attraction and didnt show enough high value). All of my friends decided we would go get some tacos (late night tacos) I insisted I'd ride with her all the way to the tacos place. We got to the tacos place, I demonstrated so HIGH VALUE, I was FUN, THE MAN, I ignored her a bit, and She ended up sayin something like "If you give me a second chance blah blah...."
In the end I drove her home and asked for her #. She grinned and gave it to me.
Now she's really into me (Have dated her 2 times now).. But that's a different story
Just to prove to you that you have to make sure you're being funny, demonstrating the highest value, getting IOI's from her and in case of rejection, do not eject right away, assess the situation and consider being perseverant. If she's not into you, just eject. Rejection can be fun sometimes
