Why sperm cell and ovum contain 23 single chromosomes?

A daughter cell with only 23 chromosomes is called a haploid cell; these are the gametes (sex cells) which are produced during meiOSis (note: this is not the same as mitosis). When a sperm & an ovum fuse at conception, the result is a single cell called a zygote, which now has 46 chromosomes - and is therefore a diploid cell.