I wouldnt want to be approached by a stranger and asked for my number. Id either think A. What a loser or B. Hes desperate.
If someone catches your eye and she signals something back then by all means work your way over and start a conversation. Talk to the person for a bit BEFORE asking for a phone number. Once you've established a conversation get a feel for her first. Talk for a bit, small chat nothing heavy.
Show some confidence not arrogance. Then if all goes well, at the end you can ask if you feel its right!
everything happens for a reason...beginning to wonder why.