Here's why:

* Sperm: The sperm cell carries a haploid nucleus, meaning it has half the number of chromosomes found in a normal cell.

* Egg: The egg cell also carries a haploid nucleus.

* Fertilization: When the sperm fertilizes the egg, their nuclei fuse together. This fusion creates a diploid zygote, with a complete set of chromosomes (half from the sperm and half from the egg).

So, while there are multiple cells involved in fertilization, only two nuclei are directly combined to form the zygote.