Why is the number of chromosomes an organisms has an even number?
The reason why the majority of organisms have an even number of chromosomes is because chromosomes are in pairs. A human, for instance, will have half its chromosomes from the father, and half from its mother.
There are exceptions to the rule. For instance, an individual with Down Syndrome will have 47 chromosomes instead of 46, because they have trisomy 21 (three copies of the 21st chromosome, instead of just two).
Another exception would be polyploidy , which occurs when organisms have more pairs of chromosomes than a diploid cell does.
Below is a picture to help visualize polyploidy. An example of a haploid cell would be a gamete (a sperm cell, for instance), and a diploid cell would be a skin cell of a person with 46 chromosomes.