How long to wait for a guy to call after you give him your number?
For most of the men that I've spoken to, three days tends to be the golden rule. It's long enough to make a girl sweat a little, but not so long that she'd forget ever having met him.
Should I text a guy who gave me his number?
Just text him saying "Hey it's *your name* :)". Text him whenever! He clearly wants to hear from you if he gave you his number. Identify yourself so he knows whose number it is, and make as much of a statement as you're willing to commit to.
Why would he give me his number?
There are many reasons a guy gives out his number, but the main reason is that he wants to hear from you. That said, he may not be that into you. He may give you his number so he can measure if you're really interested.. aka you'll call if you are.. so he can have a more “sure thing”.