How does a fertilized egg form 23 pairs of chromosomes?
After fertilisation the zygote has two copies of each of the chromosomes (not worrying about sex chromosomes). When each of these is replicated the cells will have chromosome pairs, which will then divide in mitosis.

The classical chromosome image of pairs of X-shaped chromosomes relates to the situation at mitotic metaphase when each of the chromosomes has replicated and consists of two sister chromatids. As anaphase proceeds the splitting of the centromeres means that each of the sister chromatids becomes a chromosome, and each cell ends up with a pair of each chromosome type, just like a zygote.