Like anything else, this is a numbers game. Attractiveness, confidence, etc. obviously increase chances of getting legit responses, but keep in mind that luck plays a large role in any game.
This is totally wrong these are the true factors that get results when approaching a girl and asking for her phone number:
1. Size, make your self look huge. Pin stripes, big hair, and layers like just as many layers of clothes as you can handle. This works best during the summer time if you are wearing ten layers of clothing and her boyfriend only wears 8, she's all yours.
2. Talk loud and talk fast, this is pretty obvious. The loudest man is always the most liked man. Talk fast because if you talk slow women have tendency to think about what you are saying and focus on things, sadly they aren't smart so it'll fry their brains so if you keep a steady stream of words going they'll have issues picking out one to dwell on.
3. Be aggressive, snarl at the women, get into her personal space. Get at least 3 inches away from her face. Flapping your arms like an angry bird doesn't hurt either.
Just because you are trying to get numbers doesn't mean I has to be a numbers game follow these three simple rules and I promise you'll be wandering around the bone-zone ASAP.