How to ask girl for number?
I assume you want ask number for dating purpose. 

I don't think this is a Chinese language problem. The difficulty is not how to ask but when to ask.

"number" should be translated into "号" but not "数" here.

You may say "我能留一下你的手机号吗？"(May I take down your phone number?). However, the more popular way in China is to ask their to add your WeChat(http://www.wechat.com/en/).