It depends on the situation - I would have a conversation with her and see what way it was going - I would give her my number but keep an eye on the communication between us, if I felt that she was more into it than me I would say because I have been in her situation before and I think I would have appreciated the honesty of someone saying I just don't like you that way instead of rejecting you straight off or ghosting you.
Even if she was completely unattractive, I'd still be inclined to accept her request and give her my number. Nothing wrong with having purely platonic relationships with members of the opposite sex, expanding one's social circle.
Really depends on why she wants my number. So I would ask her why she wants it. If she said it's because she finds me attractive, wants to date or to get to know me better; it would be a no. I don't date and don't really have time or interest in women who want that.
No I wouldn't. In fact a girl asked me for my number last week. I gave it her, we texted a few times then she went quiet for a few days. Whats point in asking for a guys number if you're not gonna keep in touch. I mean it must have taken her courage to ask me in 1st place?