I thought this post on the Pharyngula blog today very clearly explained how easily chromosome numbers can change.
Here's a situation that can lead to the formation of a new chromosome: what if there is a duplication of the centromere, rather than a gene? Remember, I told you that the centromere/kinetochore is where the cell attaches lines and motors to haul the chromosome to the appropriate daughter cell. In this case, two lines are attached; what if one tries to pull one centromere to the left, and the other tries to pull the other centromere to the right? Tug of war! The end result is that the chromosome is broken into two chromosomes.
Chromosome are disorganized filing cabinets, nothing more.
Take a quick look, you might clear up some misconceptions you didn't know you had.