I thought asking for peoples number was the norm. As long as you're not pushy about it go for it, and if she's attracted or interested she'll probably find it quite flattering, I do.
Don't be discouraged from doing it again if one girl says no or doesn't want to give out her number. Maybe when asking ask for two platforms so she can pick e.g 'can I have your number or facebook so we can carry on chatting'?. Something along those lines anyway.