This guy and I have been flirting with each other for a long timer and obviously sent the signal to each other that we are interesting in one another. He hasn't asked me for my number yet and I noticed he is shyer than he looks. So I decided to make the first move. I just don't know how.
How can I casually ask for his number without it seeming to needy or straight up?

In my opinion, straight up is probably the best way to go. A simple "Hey can I have your number? I'd like to text you and stuff." would probably work just fine. It may not be the smoothest approach, but people react to that kind of confidence. They'll only reject you if they're serious.

Tell him, "I lost my number. Can I have yours?"

If that fails, you can always fondle his backside. It really sends the message.

>>2 Good idea, but just don't do this at a funeral or in some weird situation.

Do what >>2 said or if you want some lolz do what >>3 said also keep in mind what >>4 said.

Oh, but if you don't want to do what the other 3 said, do what I would do. Just wait for the opportunity. Like if phone numbers come up/text message conversations/need to contact him for some reason simply ask, "Hey, I have your number right?" and obviously you don't so then if he realizes you don't (or doesn't) he can give it to you by simply asking if you could have it (or something along those lines). =) Good luck.

Thank you guys!! I hope everything goes smoothly!

And? The result? Gotta have something by now.