Most organisms that reproduce sexually contain an even number of chromosomes in their diploid cells because sexual reproduction involves the fusion of two haploid gametes, one from each parent, to form a diploid zygote. If one parent's gamete had an odd number of chromosomes and the other parent's gamete had an even number of chromosomes, the resulting zygote would have an odd number of chromosomes, which could lead to problems during cell division and development.
For example, during cell division, chromosomes pair up and separate into two daughter cells, with one copy of each chromosome going to each cell. If there were an odd number of chromosomes, it would be impossible for each daughter cell to receive an equal number of chromosomes, which could result in genetic abnormalities and developmental problems.
Having an even number of chromosomes in diploid cells ensures that each daughter cell receives an equal and complete set of chromosomes during cell division, which is essential for normal growth and development.