Why do humans have 46 chromosomes?
According to this article, no one knows; it's an "evolutionary mystery." As a general rule, closely related species generally have a similar number of chromosomes, and the number typically increases with the complexity of the species. However, the actual number varies wildly.

The author also points, out, 


  Each egg or sperm gets 23 chromosomes (half of each pair). Which
  chromosome they get in the pair is totally random. When you do the
  math, this comes out to 10 trillion different possible combinations.
  If we had only one pair of chromosomes, the number drops to 4.