It just means that someone is interested in you, and seen in this light, there's nothing wrong with that! And it doesn't at all oblige you to actually give the guy your phone number anyway. But guys, if she does, she wouldn't do that without presuming that there's a fair chance you're truly going to contact her... So don't let her wait for more than three days or forever (except when you've changed your mind all together the next morning, all sober again...)!
Is it offending if a guy asks you for your your number? - TIP
No, it's not offending. In fact, it's quite a flattering gesture, even if I don't like the guy in question (and the idea of me and him on a date - or worse, in some intimate position - fills me with horror). If you're not sharing any obstructing past "luggage", it's fine.