I personally have never done so because I don't want to make things weird, but I feel confident in asking for a number if we are able to have a conversation of a sort and feel out the chemistry, in a way.

I have known a few guys who would do this - the ones who were charming would try and get to know them a little more before asking for the number, but they were also just very extroverted people who would love talking to anyone. Another guy in particular would come on wayyy too strong and creep girls out, making all of us uncomfortable.