The male gametes of rice plant have 12 chromosomes in their nucleus. The chromosome number in female gamete, zygote and the cells of the seedling will be respectively : 

12, 24, 24

Chromosome number in male gamete of rice plant is n=12 therefore chromosome number in female gamete would also be 12. Zygote is diploid as it is the product of fertilization and the cells of the seeding would be meiocytes and other diploid cells. Hence, the chromosome number in both zygote and cells of seeding will be 2n=24.