Really, both humans and giraffe have 7 bones in their next. And no, the giraffe's didn't get that way by stretching.
A common misconception about the evolutionary process is that somehow frequent use of a particular muscle or appendage causes it to be passed on to the next generation. But that's foolishness.
Accidental genetic mutations caused ancestors of current giraffe to have slightly longer neck. That gave them a small survival advantage, given the environmental situation, and when they reproduced they passed that advantage (genetic mutation) on to their off-spring. But if the advantage didn't persist, the long neck would have disappeared. Don't believe that? Check out the Peppered Moth. Its colors changed from white to black because the local trees became covered with coal dust.