An unfertilized egg cell has six chromosomes. After fertilization, how many chromosomes should the zygote have?

12 chromosomes, because the basic (haploid) number of chromosomes is 6 (n: 6)

Explanation:

The unfertilized egg contains a basic (haploid) number of chromosomes (n), being in this case 6. In humans, for example, the gamete haploid cells have a basic n content of 23 chromosomes, while in somatic cells this DNA amount is duplicated (2n), being therefore 46 chromosomes that are organized into 22 pairs of autosomal chromosomes and two sex chromosomes (X and Y).